This is a total conversion of SciTECO to plain C (GNU C11).
The chance was taken to improve a lot of internal datastructures,
fix fundamental bugs and lay the foundations of future features.
The GTK user interface is now in an useable state!
All changes have been squashed together.

The language itself has almost not changed at all, except for:

* Detection of string terminators (usually Escape) now takes
  the string building characters into account.
  A string is only terminated outside of string building characters.
  In other words, you can now for instance write
  I^EQ[Hello$world]$
  This removes one of the last bits of shellisms which is out of
  place in SciTECO where no tokenization/lexing is performed.
  Consequently, the current termination character can also be
  escaped using ^Q/^R.
  This is used by auto completions to make sure that strings
  are inserted verbatim and without unwanted sideeffects.
* All strings can now safely contain null-characters
  (see also: 8-bit cleanliness).
  The null-character itself (^@) is not (yet) a valid SciTECO
  command, though.

An incomplete list of changes:

* We got rid of the BSD headers for RB trees and lists/queues.
  The problem with them was that they used a form of metaprogramming
  only to gain a bit of type safety. It also resulted in less
  readble code. This was a C++ desease.
  The new code avoids metaprogramming only to gain type safety.
  The BSD tree.h has been replaced by rb3ptr by Jens Stimpfle
  (https://github.com/jstimpfle/rb3ptr).
  This implementation is also more memory efficient than BSD's.
  The BSD list.h and queue.h has been replaced with a custom
  src/list.h.
* Fixed crashes, performance issues and compatibility issues with
  the Gtk 3 User Interface.
  It is now more or less ready for general use.
  The GDK lock is no longer used to avoid using deprecated functions.
  On the downside, the new implementation (driving the Gtk event loop
  stepwise) is even slower than the old one.
  A few glitches remain (see TODO), but it is hoped that they will
  be resolved by the Scintilla update which will be performed soon.
* A lot of program units have been split up, so they are shorter
  and easier to maintain: core-commands.c, qreg-commands.c,
  goto-commands.c, file-utils.h.
* Parser states are simply structs of callbacks now.
  They still use a kind of polymorphy using a preprocessor trick.
  TECO_DEFINE_STATE() takes an initializer list that will be
  merged with the default list of field initializers.
  To "subclass" states, you can simply define new macros that add
  initializers to existing macros.
* Parsers no longer have a "transitions" table but the input_cb()
  may use switch-case statements.
  There are also teco_machine_main_transition_t now which can
  be used to implement simple transitions. Additionally, you
  can specify functions to execute during transitions.
  This largely avoids long switch-case-statements.
* Parsers are embeddable/reusable now, at least in parse-only mode.
  This does not currently bring any advantages but may later
  be used to write a Scintilla lexer for TECO syntax highlighting.
  Once parsers are fully embeddable, it will also be possible
  to run TECO macros in a kind of coroutine which would allow
  them to process string arguments in real time.
* undo.[ch] still uses metaprogramming extensively but via
  the C preprocessor of course. On the downside, most undo
  token generators must be initiated explicitly (theoretically
  we could have used embedded functions / trampolines to
  instantiate automatically but this has turned out to be
  dangereous).
  There is a TECO_DEFINE_UNDO_CALL() to generate closures for
  arbitrary functions now (ie. to call an arbitrary function
  at undo-time). This simplified a lot of code and is much
  shorter than manually pushing undo tokens in many cases.
* Instead of the ridiculous C++ Curiously Recurring Template
  Pattern to achieve static polymorphy for user interface
  implementations, we now simply declare all functions to
  implement in interface.h and link in the implementations.
  This is possible since we no longer hace to define
  interface subclasses (all state is static variables in
  the interface's *.c files).
* Headers are now significantly shorter than in C++ since
  we can often hide more of our "class" implementations.
* Memory counting is based on dlmalloc for most platforms now.
  Unfortunately, there is no malloc implementation that
  provides an efficient constant-time memory counter that
  is guaranteed to decrease when freeing memory.
  But since we use a defined malloc implementation now,
  malloc_usable_size() can be used safely for tracking memory use.
  malloc() replacement is very tricky on Windows, so we
  use a poll thread on Windows. This can also be enabled
  on other supported platforms using --disable-malloc-replacement.
  All in all, I'm still not pleased with the state of memory
  limiting. It is a mess.
* Error handling uses GError now. This has the advantage that
  the GError codes can be reused once we support error catching
  in the SciTECO language.
* Added a few more test suite cases.
* Haiku is no longer supported as builds are instable and
  I did not manage to debug them - quite possibly Haiku bugs
  were responsible.
* Glib v2.44 or later are now required.
  The GTK UI requires Gtk+ v3.12 or later now.
  The GtkFlowBox fallback and sciteco-wrapper workaround are
  no longer required.
* We now extensively use the GCC/Clang-specific g_auto
  feature (automatic deallocations when leaving the current
  code block).
* Updated copyright to 2021.
  SciTECO has been in continuous development, even though there
  have been no commits since 2018.
* Since these changes are so significant, the target release has
  been set to v2.0.
  It is planned that beginning with v3.0, the language will be
  kept stable.

 * on MSVCRT/MinGW, space allocated with alloca()/g_newa() was apparently
   freed once the first exception was caught.
   This prevented the proper destruction of local Q-Reg tables and
   broke the Windows port.
 * Since all alternatives to alloca() like VLAs are not practical,
   the default Q-Register initialization has been moved out of the
   QRegisterTable constructor into QRegisterTable::insert_defaults().
 * The remaining QRegisterTable initialization and destruction is
   very cheap, so we simply reserve an empty QRegisterTable for
   local registers on every Execute::macro() call.
   The default registers are only initialized when required, though.
 * All of this has to change anyway once we replace the
   C++ call-stack approach to macro calls with our own macro
   call frame memory management.

 * we were basing the glib allocators on throwing std::bad_alloc just like
   the C++ operators. However, this always was unsafe since we were throwing
   exceptions across plain-C frames (Glib).
   Also, the memory vtable has been deprecated in Glib, resulting in
   ugly warnings.
 * Instead, we now let the C++ new/delete operators work like Glib
   by basing them on g_malloc/g_slice.
   This means they will assert and the application will terminate
   abnormally in case of OOM. OOMs cannot be handled properly anyway, so it is
   more important to have a good memory limiting mechanism.
 * Memory limiting has been completely revised.
   Instead of approximating undo stack sizes using virtual methods
   (which is unprecise and comes with a performance penalty),
   we now use a common base class SciTECO::Object to count the memory
   required by all objects allocated within SciTECO.
   This is less precise than using global replacement new/deletes
   which would allow us to control allocations in all C++ code including
   Scintilla, but they are only supported as of C++14 (GCC 5) and adding compile-time
   checks would be cumbersome.
   In any case, we're missing Glib allocations (esp. strings).
 * As a platform-specific extension, on Linux/glibc we use mallinfo()
   to count the exact memory usage of the process.
   On Windows, we use GetProcessMemoryInfo() -- the latter implementation
   is currently UNTESTED.
 * We use g_malloc() for new/delete operators when there is
   malloc_trim() since g_slice does not free heap chunks properly
   (probably does its own mmap()ing), rendering malloc_trim() ineffective.
   We've also benchmarked g_slice on Linux/glib (malloc_trim() shouldn't
   be available elsewhere) and found that it brings no significant
   performance benefit.
   On all other platforms, we use g_slice since it is assumed
   that it at least does not hurt.
   The new g_slice based allocators should be tested on MSVCRT
   since I assume that they bring a significant performance benefit
   on Windows.
 * Memory limiting does now work in batch mode as well and is still
   enabled by default.
 * The old UndoTokenWithSize CRTP hack could be removed.
   UndoStack operations should be a bit faster now.
   But on the other hand, there will be an overhead due to repeated
   memory limit checking on every processed character.

 * it turns out that option-like arguments could not be reliably passed to
   SciTECO scripts for two reasons:
   a) "--" arguments are not removed from argv by GOption if it detects
      and following option-like argument.
      "--" would thus be passed as a script argument which will disable
      option parsing in scripts that interpret "--".
   b) A script run via the Hash-Bang line "#!...sciteco -m" would
      require an explicit "--" to turn of GOption parsing.
      However it is __impossible__ to insert after the script file name
      on UNIX.
 * Therefore, SciTECO now removes leading "--" arguments left over by GOption.
 * If possible (Glib >= 2.44), option parsing is performed in strict POSIX
   mode which inhibits parsing after the first non-option argument.
   This reduces the number of cases where an explicit "--" is required.
 * --mung no longer takes an argument. Instead, the first non-option argument
   is expected to be the script file name.
   This looks weird at first but is more consistent with how other interpeters
   work. Once we revise argument passing to scripts, the script name can also
   be passed to the script which is more consistent with it being the first
   non-option argument.
   Also, with strict POSIX parsing, this fixed Hash-Bang lines since
   the script file name constructed by the kernel will automatically switch
   off option parsing, passing all option-like script arguments uninterpreted
   to the script.
 * Since we're supporting Glib < 2.44, the Hash-Bang lines are still broken
   for certain builds.
   Therefore, a wrapper script is installed to libexecdir (it never has to be
   executed by users and Hash-Bang lines need absolute paths anyway) which
   transparently inserts "--" into the SciTECO command line and should be used
   as the interpreter in portable SciTECO scripts.
   The wrapper script is generated and points to the exact SciTECO binary
   installed. This is important when doing parallel installs of Curses and Gtk
   binaries since each one will get its own working wrapper script.
   The wrapper-script workaround can be removed once we depend on Glib >= 2.44
   (some day...).
 * The default /usr/bin/env Hash-Bang lines are no longer used in the
   scripts since they are broken anyway (UNIX incl. Linux cannot pass
   multiple arguments to the interpreter!).
   Scripts that get installed will get a fixed-up Hash-Bang line referring
   to the installed SciTECO binary anyway.
 * Interface::main() has been renamed to Interface::init() and is optional
   now. The Interface::main() method was introduced because of the misconception
   that interfaces will find their options in the argv array and have to do
   their own parsing.
   This is wrong, since their option group already cares about parsing.
   Therefore, gtk_init() does not have to called explicitly, too.

 * The default command interpreter will thus be inherited from
   the operating system. In the case of UNIX from the user's
   passwd entry.
   E.g. if bash is used, bash extensions can be used immediately
   if flag 128 is not set in the ED flags.
 * On DOS-like systems there are also alternative interpreters
   (e.g. 4NT, 4OS2) that are configurable now.
 * At least on UNIX with $SHELL it is not guaranteed that
   the interpreter supports the standard command line arguments
   like "-c". If they don't, this will cause problems with EC.
   Since $SHELL is mapped to a Q-Register, it can however
   always be easily customized for SciTECO sessions in the
   user's .teco_ini.

 * acts like exit(3) -- ie. the program is terminated
   immediately but the quit hook (aka SciTECO's atexit()
   handlers) will still run.
 * for "compatibility" with classic TECOs.
   Can also be used as a shorter variant of "-EX$$"
   but working from every macro level.
 * disallowed in interactive mode to avoid typing it
   accidentally.

 * Haiku can be handled like UNIX in most respects
   since it is POSIX compliant, has a UNIX-like terminal
   emulator and uses ncurses.
 * still the Glib platform macro is G_OS_HAIKU instead of
   G_OS_UNIX, so the preprocessor conditionals had to be adapted.
 * the only functional difference between a Haiku and UNIX build
   is the default SCITECOCONFIG path.
   We use the config path returned by Glib instead of $HOME,
   so .teco_ini will be in ~/config/settings on Haiku.
   Other UNIX ports appear to use the same conventions.
 * Some Haiku-specific restrictions still apply:
   * Haiku's terminal is xterm-compatible, but only supports
     8 colors. Therefore only the terminal.tes color scheme
     can be used and the terminal must be set up to
     "Use bright instead of bold text".
   * The terminal has artifacts. This appears to be a Haiku
     bug and affects other curses applications as well.
   * GTK is yet unsupported on Haiku, so there may never be
     a GUI port (unless someone writes a QT GUI for SciTECO).
   * SciTECO cannot be built with the legacy gcc2 used for
     BeOS compatibility on Haiku. This would require too many
     changes for an obsolete platform.
     BeOS and the x86_gcc2 platform of Haiku will therefore
     never be supported.
     The PPC and ARM platforms of Haiku should work but are untested.
 * a HaikuPorts recipe will be provided for the next regular
   SciTECO release. This should hopefully allow installation via
   HaikuDepot.

 * ^FCLOSE is inserted when the "Close" key is pressed.
   It is used by the GTK+ UI to deliver window close requests
   and SIGTERM occurrences.
   (this replaces the "Break" key used before in the GTK+ UI).
 * The default action of ^FCLOSE is to quit SciTECO, therefore
   window closing is possible even in --no-profile mode for instance.
 * fixed a minor memleak in Cmdline::fnmacro()
 * added ^FCLOSE implementation to fnkeys.tes to insert EX.
   This currently has the disadvantage of overwriting
   the error message with syntax errors if there are modified buffers
   but it will at least not close the window if there are modified
   buffers.
 * SIGTERM will now be similar to SIGINT by default instead of
   terminating SciTECO right away.
 * the GTK+ UI handles SIGTERM by emulating the "close" key while
   still interrupting like SIGINT.
 * GTK+: SIGTERM and ^C will interrupt by sending SIGINT to the
   entire process group instead of simply setting `sigint_occurred`.
   This fixes interrupting EC and EG commands with long-running
   or hanging programs and is relevant to the solution of #4.