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  The BSD tree.h has been replaced by rb3ptr by Jens Stimpfle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

 * characters rubbed out are not totally removed from the command line,
   but only from the *effective* command line.
 * The rubbed out command line is displayed after the command line cursor.
   On Curses it is grey and underlined.
 * When characters are inserted that are on the rubbed out part of the command line,
   the cursor simply moves forward.
   NOTE: There's currently no immediate editing command for reinserting the
   next character/word from the rubbed out command line.
 * Characters resulting in errors are no longer simply discarded but rubbed out,
   so they will stay in the rubbed out part of the command line, reminding you
   which character caused the error.
 * Improved Cmdline formatting on Curses UI:
   * Asterisk is printed bold
   * Control characters are printed in REVERSE style, similar to what
     Scinterm does. The controll character formatting has thus been moved
     from macro_echo() in cmdline.cpp to the UI implementations.
 * Updated the GTK+ UI (UNTESTED): I did only, the most important API
   adaptions. The command line still does not use any colors.
 * Refactored entire command line handling:
   * The command line is now a class (Cmdline), and most functions
     in cmdline.cpp have been converted to methods.
   * Esp. process_edit_cmd() (now Cmdline::process_edit_cmd()) has been
     simplified. There is no longer the possibility of a buffer overflow
     because of static insertion buffer sizes
   * Cleaned up usage of the cmdline_pos variable (now Cmdline::pc) which
     is really a program counter that used a different origin as macro_pc
     which was really confusing.
   * The new Cmdline class is theoretically 8-bit clean. However all of this
     will change again when we introduce Scintilla views for the command line.
 * Added 8-bit clean (null-byte aware) versions of QRegisterData::set_string()
   and QRegisterData::append_string()

the QUIT hook is actually not that trivial and required
some architectural changes.

First, the QUIT hook execution and any error that might
occurr cannot always be attached to an existing error stack
frame. Thereforce, to give a stack frame for QUIT hooks and
to improve the readability of error traces for ED hooks in general,
a special EDHookFrame is added to every ED hook execution error.

Secondly, since QUIT hooks can themselves throw errors, we cannot
run it from an atexit() handler. Instead it's always called manually
before __successful__ program termination. An error in a QUIT hook
will result in a failure return code nevertheless.

Thirdly, errors in QUIT hooks should not prevent program termination
(in interactive mode), therefore they are only invoked from main()
and always in batch mode. I.e. if the interactive mode is terminated
(EX$$), SciTECO will switch back to batch mode and run the QUIT
hook there. This is also symmetric to program startup, which is
always in batch mode.

This means that Interface::event_loop() no longer runs indefinitely.
If it returns, this signals that the interface shut down and
batch mode may be restored by SciTECO.

 * without the one-view-per-buffer designs, many Scintilla send message (SSM)
   calls could be inlined
 * with the new design, this was no longer possible using the abstract
   base classes. the CRT pattern allows inlining again but introduces
   a strange level of code obscurity.
 * tests suggest that at high optimization levels, the one-view-per-buffer
   design and the CRT pattern reduces typical macro runtimes by 30%
   (e.g. for symbols-extract.tes).
 * only updated the NCurses UI for the time being

 * the GError expection has been renamed to GlibError, to avoid
   nameclashes when working from the SciTECO namespace