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  The BSD tree.h has been replaced by rb3ptr by Jens Stimpfle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

 * some code simplifications
 * it now supports command line arguments via getopt.tes.
 * the -C flag enabled C/C++ mode.
   By default tedoc parses SciTECO code which means it can be used
   to document macro packages as well.
 * Therefore it is installed as a separate tool now.
   It may be used as a Groff preprocessor for third-party macro
   authors to generate (wo)man pages.
 * there's a man page tedoc.tes(1)
 * The troff placeholder macro is now called ".TEDOC".
 * Help topics can now be specified after the starting comment /*$ or !*$.
   Topics have been defined for all built-in commands.

 * globbing is fnmatch(3) compatible, now on every supported platform.
 * which means that escaping of glob patterns is possible now.
   ^ENq has been introduced to ease this task.
 * This finally allows you to pass unmodified filenames to EB.
   Previously it was impossible to open file names containing glob wildcards.
 * this was achieved by moving from GPattern to GRegex as the underlying
   implementation.
 * The glob pattern is converted to a regular expression before being
   compiled to a GRegex.
   This turned out to be trickier than anticipated (~140 lines of code)
   and has a runtime penalty of course (complexity is O(2*n) over the
   pattern length).
   It is IMHO still better than the alternatives, like importing
   external code from libiberty, which is potentially non-cross-platform.
 * Using GRegex also opens the potential of supporting brace "expansions"
   later in the form of glob pattern constructs
   (they won't actually expand but match alternatives).
 * is_glob_pattern() has been simplified and moved to Globber::is_pattern().
   It makes sense to reuse the Globber class namespace instead of using
   plain functions for functions working on glob patterns.
 * The documentation has a new subsection on glob patterns now.
 * Testsuite extended with glob pattern test cases

 * 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

 * EN may now be used for matching file names (similar to fnmatch(3)).
   This is used to check the current buffers file extension in the
   lexer configuration macros instead of using expensive Q-Register
   manipulations.
   This halves the overall startup time - it is now acceptable even
   with the current amount of lexer configurations.
 * EN may now be used for checking file types.
   session.tes has been simplified.
 * BREAKS macro portability (EN now has 2 string arguments).
 * The Globber class has been extended to allow filtering of
   glob results by file type.

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

 * Globbing without directory (e.g. EN*.cpp$) introduced a "./" into the
   expanded file names.
   It no longer does that.
 * The expanded file names will have the exact same directory component
   (if any) as the glob pattern.
   So on Windows, the directory separators in the list of expanded files
   is exactly as the user requested.
 * Also fixes lexers.tes on Windows because the script assumes forward
   slashes.