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  The BSD tree.h has been replaced by rb3ptr by Jens Stimpfle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

 * the old implementation tried to avoid template programming by
   making the entry comparison function virtual.
 * The new RBTree implementation takes a template argument with the
   implementation of RBEntry. It is now partially conventional
   that the template argument must be actually derived from RBTree::RBEntry
   and must define a "compare" method.
 * As an advantage we now get static polymorphism (avoiding virtual
   calls and allowing for more compiler optimizations) and the
   the RBEntry implementation no longer has to be virtual.
 * The only RB-Trees actually used are string-keyed, though.
   Therefore there's a common base class RBTreeString now which
   defines two synonymous "key" and "name" attributes.
   * The entry base class RBEntryString is virtual again because
     we do not want to propagate the RBEntryType template parameter
     even further and the RBTree base class needs to destroy
     entries.
     This might be avoided by not defining a RBTree::clear() method,
     leaving this task to the implementations.
     At least QRegisters have to be virtual, though.
   * RBTreeString only depends on the strcmp() and strncmp() functions
     used now and only case-sensitive and case-insensitive versions
     are actually required, so we instantiate these templates statically
     in rbtree.cpp.
     This means there are still only two instantiations of the RBTree
     in the binary.
   * RBTreeString defines convenient wrappers for find() and nfind()
     to look up by string.
     This uses the RBEntryString base class, so no allocations whatsover
     are required for lookups and less space is wasted on the call stack.
   * A RBEntryOwnString base class is also provided which frees the
     implementations from memory managing the tree keys.
   * RBTreeString can now be used to add other common functionality
     like auto-completions for Q-Registers, goto labels and help topics.
 * some minor optimizations
 * updated TODO

 * 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.

 * also improved performance of String::append() by using g_realloc()
 * added String::append() variant for non-null-terminated strings

normally, since SciTECO is not a library, this is not strictly
necessary since every library should use proper name prefixes
or namespaces for all global declarations to avoid name clashes.

However

 * you cannot always rely on that
 * Scintilla does violate the practice of using prefixes or namespaces.
   The public APIs are OK, but it does define global functions/methods,
   e.g. for "Document" that clashed with SciTECO's "TECODocument" class at
   link-time.
   Scintilla can put its definitions in a namespace, but this feature
   cannot be easily enabled without patching Scintilla.
 * a "SciTECO" namespace will be necessary if "SciTECO" is ever to be
   turned into a library. Even if this library will have only a C-linkage
   API, it must ensure it doesn't clutter the global namespace.

So the old "TECODocument" class was renamed back to "Document"
(SciTECO::Document).

 * specifications resulted in runtime errors (unexpected exception) when bad_alloc ocurred
 * specs should be used scarcely: only when the errors that may be thrown are all known
   and for documentary purposes