* fixed some Debian lintian warnings

* Previous Scintilla version was 3.6.4 and Scinterm was 1.7 (with lots of custom patches).
  All of the patches are now either irrelevant or have been merged upstream.
* Since Scintilla 5 requires C++17, this increases the minimum GCC version at least
  to 5.0. We may actually require even newer versions.
* I could not upgrade the scintilla-mirror (which was imported from Mercurial),
  so the old sciteco-dev branch was renamed to sciteco-dev-pre-v2.0.0,
  master was deleted and I reimported the entire Scintilla repo using
  git-remote-hg.
  This means that scintilla-mirror now contains two entirely separate trees.
  But it is still possible to clone old SciTECO repos.
* The strategy/workflow of maintaining hotfix branches on scintilla-mirror has been changed.
  Instead of having one sciteco-dev branch that is rebased onto new Scintilla upstream
  releases and tagging SciTECO releases in scintilla-mirror (to keep the commits referenced),
  we now create a branch for every Scintilla version we are based on (eg. sciteco-rel-5-1-3).
  This branch is never rebased or deleted. Therefore, we are guaranteed to be able to
  clone arbitrary SciTECO repo commits - not only releases.
  Releases no longer have to be tagged in scintilla-mirror.
  On the downside, fixup commits may accumulate in these new branches.
  They can only be squashed once a new branch for a new Scintilla release is created
  (e.g. by cherry-picking followed by rebase).
* Scinterm does no longer have to reside in the Scintilla subdirectory,
  so we added it as a regular submodule.
  There are no more recursive submodules.
  The Scinterm build system has not been improved at all, but we use
  a trick based on VPATH to build Scinterm in scintilla/bin/.
* Scinterm is now in Git and we reference the upstream repo for the
  time being.
  We might mirror it and apply the same branching workflow as with Scintilla
  if necessary.
  The scinterm-mirror repository still exists but has not been touched.
  We will also have to rewrite its master branch as it was a non-reproducible
  Mercurial import.
* Scinterm now also comes with patches for Scintilla which we simply applied
  on our sciteco-rel-5-1-3 branch.
* Scintilla 5 outsourced its lexers into the Lexilla project.
  We added it as yet another submodule.
* All submodules have been moved into contrib/.
* The Scintilla API for setting lexers has consequently changed.
  We now have to call SCI_SETILEXER(0, CreateLexer(name)).
  As I did not want to introduce a separate command for setting lexers,
  <ES> has been extended to allow setting lexers by name with the SCI_SETILEXER
  message which effectively replaces SCI_SETLEXERLANGUAGE.
* The lexer macros (SCLEX_...) no longer serve any purpose - they weren't used
  in the SciTECO standard library anyway - and have consequently been removed
  from symbols-scilexer.c.
  The style macros from SciLexer.h (SCE_...) are theoretically still useful - even
  though they are not used by our current color schemes - and have therefore been
  retained. They can be specified as wParam in <ES>.
* <ES> no longer allows symbolic constants for lParam.
  This never made any sense since all supported symbols were always wParam.
* Scinterm supports new native cursor modes.
  They are not used for the time being and the previous CARETSTYLE_BLOCK_AFTER
  caret style is configured by default.
  It makes no sense to enable native cursor modes now since the
  command line should have a native cursor but is not yet a Scintilla view.
* The Scintilla upgrade performed much worse than before,
  so some optimizations will be necessary.

* 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.)

* Added a Freedesktop file - only as part of the debian package yet.
  `make install` won't install a Desktop file since it would have to be
  generated.
* Just like when installing manually, you can have a Curses and Gtk installation
  side by side using the same .teco_ini.
* Common data between the Curses UI (sciteco-curses) and Gtk UI are in a new
  architecture-independant package sciteco-common.
* The Gtk+ binaries are prefixed with `g` (gsciteco, gtedoc.tes, ggrosciteco.tes).
* Debian source and binary packages can be built using `./distribute.mk debian`
  as usual.
  It should also be possible to push everything to the PPA for the next release,
  although that is not yet tested.

* required a new changelog entry since the target release is now v2.0.0
* Glib dependency bumped

 * the new "?" (help) command can be used to look up
   help topics.
 * help topics are index from $SCITECOPATH/women/*.woman.tec
   files.
 * looking up a help topic opens the corresponding "womanpage"
   and jumps to the position of the topic (it acts like an anchor
   into the document).
 * styling is performed by *.woman.tec files.
 * Setting up the Scintilla view and munging the *.tec file
   is performed by the new "woman.tes" lexer.
   On supporting UIs (Gtk), womanpages are shown in a variable-width
   font.
 * Woman pages are usually not hand-written, but generated from manpages.
   A special Groff post-processor grosciteco has been introduced for this
   purpose. It is much like grotty, but can output SciTECO macros for styling
   the document (ie. the *.woman.tec files).
   It is documented in its own man-page.
 * grosciteco also introduces sciteco.tmac - special Troff macros
   for controlling the formatting of the document in SciTECO.
   It also defines .SCITECO_TOPIC which can be used to mark up
   help topics/terms in Troff markup.
 * Woman pages are generated/formatted by grosciteco at compile-time, so
   they will work on platforms without Groff (ie. as on windows).
 * Groff has been added as a hard compile-time requirement.
 * The sciteco(1) and sciteco(7) man pages have been augmented with
   help topic anchors.

 * it also sets the compatibility level to 7 which was required
   when building for Ubuntu Lucid. This version cannot be supported
   however since its libglib version is too old.