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  The BSD tree.h has been replaced by rb3ptr by Jens Stimpfle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.

 * 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.

 * mapped to different registers beginning with "~"
 * on supported platforms accessing the clipboard is as easy as
   X~ or G~.
   Naturally this also allows clipboards to be pasted in
   string arguments/insertions (^EQ~).
 * Currently, Gtk+, PDCurses and ncurses/XTerm are supported.
   For XTerm clipboard support, users must set 0,256ED to enable
   it since we cannot check for XTerm window ops programmatically
   (at least without libX11).
 * When clipboard regs exist, the clipboard can also be deemed functional.
   This allows macros to fall back to xclip(1) if necessary.
 * EOL handling has been moved into a new file eol.c and eol.h.
   EOL translation no longer depends on GIOChannels but can be
   memory-backed as well.

 * the old implementation was totally broken, which was to be expected
 * we can at least provide a version that always returns an absolute
   path, even though it does not canonicalizes
 * fixes e.g. Haiku builds for the time being.
   Haiku however is mostly POSIX compliant and could be handled
   like UNIX.
 * simplified the UNIX implementation of get_absolute_path()

 * expands to the value of $HOME (the env variable instead of
   the register which currently makes a slight difference).
 * supported for tab-completions
 * supported for all file-name accepting commands.
   The expansion is done centrally in StateExpectFile::done().
   A new virtual method StateExpectFile::got_file() has been
   introduced to pass the expanded/processed file name to
   command implementations.
 * sciteco(7) has been updated: There is now a separate section
   on file name arguments and file name handling in SciTECO.
   This information is important but has been scattered across
   the document previously.
 * optimized is_glob_pattern() in glob.h

 * FG stands for "Folder Go"
 * FG behaves similar to a Unix shell `cd`.
   Without arguments, it changes to the $HOME directory.
 * The $HOME directory was previously only used by $SCITECOCONFIG on Unix.
   Now it is documented on its own, since the HOME directory should also
   be configurable on Windows - e.g. to adapt SciTECO to a MinGW or Cygwin
   installation.
   HOME is initialized just like the other environment variables.
   This also means that now, the $HOME Q-Register is always defined
   and can be used by platform-agnostic macros.
 * FG uses a new kind of tab-completion: for directories only.
   It would be annoying to complete the FG command after every
   directory, so this tab-completion does not close the command
   automatically. Theoretically, it would be possible to close
   the command after completing a directory with no subdirectories,
   but this is not supported currently.
 * Filename arguments are no longer completed with " " if {} escaping
   is in place as this brings no benefit. Instead no completion character
   is inserted for this escape mode.
 * "$" was mapped to the current directory to support an elegant way to
   insert/get the current directory.
   Also this allows the idiom "[$ FG...new_dir...$ ]$" for changing
   the current directory temporarily.
 * The Q-Register stack was extended to support restoring the string
   part of special Q-Registers (that overwrite the default functionality)
   when using the "[$" and "]$" commands.
 * fixed minor typos (american spelling)

 * activated via bit 4 of the ED flag (enabled by default)
 * automatic EOL guessing on file loading and translation to LFs.
   * works with files that have inconsistent EOL sequences.
 * automatic translation to original EOL sequences on file saving
   * works with inconsistent EOL sequences in the buffer.
     This should usually not happen if the file was read in with
     automatic EOL translation enabled.
 * also works with the EC and EG commands
 * performance is OK, depending on the file being translated.
   When reading files with UNIX EOLs, the overhead is minimal
   typically-sized files. For DOS EOLs the overhead is larger
   but still acceptable.
 * Return (line feed) is now an immediate editing command.
   This centralizes EOL sequence insertion.
   Later, other features like auto-indent could be added to
   the editing command.
 * get_eol() has been moved to main.cpp (now called
   get_eol_seq()
 * Warn if file ownership could not be preserved when
   saving files.
 * IOView has been almost completely rewritten based
   on GIOChannels. The EOL translation code is also in IOView.

this function is very useful in other places as well
(e.g. command line tab completion)