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  The BSD tree.h has been replaced by rb3ptr by Jens Stimpfle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

 * since SCI_SETDOCPOINTER resets character representations
   (should probably be submitted as a bug to Scintilla)
   we have to reset the representations each time we load
   a q-register into the q-reg view.
 * since the SCI_SETREPRESENTION call does not do any redrawing
   (and it would be very slow if it did), the lines with control
   characters were laid out wrong (too much spaces).
   This happened when editing a q-reg or the command-line.
 * Since it is not obvious how to fix Scintilla's behaviour here,
   we work around the issue by temporarily disabling the layout
   cache.

 * it must be initialized after the UI (Interface::main), so I added
   a View::initialize() function
 * the old initialize() method was renamed to setup()
 * use a global instance of QRegister::view so it is guaranteed to
   be destroyed only after any QRegisters that could still need it
 * Document API adapted to work with ViewCurrent references

The user interface provides a Scintilla view abstraction and
every buffer is based on a view. All Q-Register strings use
a single dedicated view to save memory and initialization time
when using many string registers.

 * this means we can finally implement a working lexer configuration
   and it only has to be done once when the buffer is first added
   to the ring. It is unnecessary to magically restore the lexer
   styles upon rubout of EB (very hard to implement anyway). It
   is also not necessary to rerun the lexer configuration macro
   upon rubout which would be hard to reconcile with SciTECO's
   basic design since every side-effect should be attached to a
   character.
 * this means that opening buffers is slightly slower now
   because of the view initialization
 * on the other hand, macros with many string q-reg operations
   are faster now, since the document must no longer be changed
   on the buffer's view and restored later on.
 * also now we can make a difference between editing a document
   in a view and changing the current view, which reduces UI calls
 * the Document class has been retained as an abstraction about
   Scintilla documents, used by QRegister Strings.
   It had to be made virtual, so the view on which the document
   is created can be specified by a virtual function.
   There is no additional space overhead for Documents.

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).

 * the TECODocument::undo_edit() function is called before TECODocument::edit()
   and the Scintilla document might still be unitialized
 * fixes e.g. undoing of Xq, ^Uq on registers whose string part
   has not being used before