* Using modern GObject idioms and macros greatly reduces the necessary boilerplate code.
* The plain C versions of our GObject classes are now "final" (cannot be derived)
  This means we can hide the instance structures from the headers and avoid using
  explicit private fields.
* Avoids some deprecation warnings when building the Gtk UI.
* GOB2 is apparently no longer maintained, so this seems like a good idea in the long run.
* The most important reason however is that there is no precompiled GOB2 for Windows
  which prevents compilation on native Windows hosts, eg. during nightly builds.
  This is even more important as Gtk+3 is distributed on Windows practically
  exclusively via MSYS.
  (ArchLinux contains MinGW gtk3 packages as well, so cross-compiling from ArchLinux
  would have been an alternative.)

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  The BSD tree.h has been replaced by rb3ptr by Jens Stimpfle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

 * especially to improve building on FreeBSD 11
 * We need GNU Make, yet alone because Scintilla/Scinterm
   needs it. We now document that dependency and added
   an Autoconf check from the autoconf-archive.
   We make sure that the build process is invoked with GNU make
   by generating only GNUmakefiles.
   The Makefile.am files have not been renamed, so this
   change can be rolled back easily.
 * Some GNU-Make-specific autoreconf warnings have still been
   resolved. But not all of them, as this would have been
   unelegant and we need GNU Make anyway.
 * Declare ACLOCAL_AMFLAGS to appease autoreconf
 * Added an explicit check for C++11 from the autoconf-archives.
   In general we should support building with every C++11 compiler
   that is sufficiently GNU-like.
 * Do not use `sed` for inplace editing, as different sed-implementations
   have mutually incompatible syntax for this.
   Instead of declaring and checking a dependency on GNU sed,
   we simply use SciTECO for the editing task.
   This improves code portability on BSDs.
 * Similarily, BSD/POSIX `cmp` is supported now.
   This fixes the test suite on BSD without declaring a
   dependency on the GNU coreutils.
 * Simplified sciteco-wrapper generation.

 * it turns out that option-like arguments could not be reliably passed to
   SciTECO scripts for two reasons:
   a) "--" arguments are not removed from argv by GOption if it detects
      and following option-like argument.
      "--" would thus be passed as a script argument which will disable
      option parsing in scripts that interpret "--".
   b) A script run via the Hash-Bang line "#!...sciteco -m" would
      require an explicit "--" to turn of GOption parsing.
      However it is __impossible__ to insert after the script file name
      on UNIX.
 * Therefore, SciTECO now removes leading "--" arguments left over by GOption.
 * If possible (Glib >= 2.44), option parsing is performed in strict POSIX
   mode which inhibits parsing after the first non-option argument.
   This reduces the number of cases where an explicit "--" is required.
 * --mung no longer takes an argument. Instead, the first non-option argument
   is expected to be the script file name.
   This looks weird at first but is more consistent with how other interpeters
   work. Once we revise argument passing to scripts, the script name can also
   be passed to the script which is more consistent with it being the first
   non-option argument.
   Also, with strict POSIX parsing, this fixed Hash-Bang lines since
   the script file name constructed by the kernel will automatically switch
   off option parsing, passing all option-like script arguments uninterpreted
   to the script.
 * Since we're supporting Glib < 2.44, the Hash-Bang lines are still broken
   for certain builds.
   Therefore, a wrapper script is installed to libexecdir (it never has to be
   executed by users and Hash-Bang lines need absolute paths anyway) which
   transparently inserts "--" into the SciTECO command line and should be used
   as the interpreter in portable SciTECO scripts.
   The wrapper script is generated and points to the exact SciTECO binary
   installed. This is important when doing parallel installs of Curses and Gtk
   binaries since each one will get its own working wrapper script.
   The wrapper-script workaround can be removed once we depend on Glib >= 2.44
   (some day...).
 * The default /usr/bin/env Hash-Bang lines are no longer used in the
   scripts since they are broken anyway (UNIX incl. Linux cannot pass
   multiple arguments to the interpreter!).
   Scripts that get installed will get a fixed-up Hash-Bang line referring
   to the installed SciTECO binary anyway.
 * Interface::main() has been renamed to Interface::init() and is optional
   now. The Interface::main() method was introduced because of the misconception
   that interfaces will find their options in the argv array and have to do
   their own parsing.
   This is wrong, since their option group already cares about parsing.
   Therefore, gtk_init() does not have to called explicitly, too.

 * makes sense since it already extracted information from ./configure
   that is usually substituted.
 * it already had to be run from a configured build directory
 * it required the source tree directory, which had to be overwritten
   on the Make command line when using an out-of-source build dir.
   This is no longer necessary.
 * It is still a stand-alone Makefile to keep it isolated from the main
   build system, although it could certainly be translated to Automake.
 * the generated file will now be called distribute.mk to signify
   that it is a Makefile.

 * Autotest ships with Autoconf, so it's available already
   and relatively easy to integrate into an Autotools package.
 * This is attached to `make check` using some Automake magic.
 * The test suite will only call the built SciTECO for the time being.
   But using tests/Makefile.am, custom programs could be easily
   built.
 * Since it uses the target sciteco, it cannot work in cross-compile
   environments.
 * The test suite tests/testsuite.at should be used for regression
   tests at least: Whenever there is a bug, a test case should be
   added to testsuite.at.
   Later this might be split up into multiple includes for regressions
   other tests.

these should be put by the user in his/her global or repository-specific
ignore patterns

 * we cannot use weak symbols in MinGW, so we avoid init_priority for symbol
   initialization by compiling the empty definitions into
   sciteco-minimal but the real ones into sciteco
   (had to add new file symbols-minimal.cpp)
 * this fixes compilation/linking on LLVM Clang AND Dragonegg
   since their init_priority attribute is broken!
   this will likely be fixed in the near future but broken versions
   will be around for some time