* g_utf8_get_char_validated() returns -2 for null-bytes (sometimes!?)

This is a purely cosmetic change.

* this works by embedding the SciTECO parser and driving it always (exclusively)
  in parse-only mode.
* A new teco_state_t::style determines the Scintilla style for any character
  accepted in the given state.
* Therefore, the SciTECO lexer is always 100% exact and corresponds to the current
  SciTECO grammer - it does not have to be maintained separately.
  There are a few exceptions and tweaks, though.
* The contents of curly-brace escapes (`@^Uq{...}`) are rendered as ordinary
  code using a separate parser instance.
  This can be disabled with the lexer.sciteco.macrodef property.
  Unfortunately, SciTECO does not currently allow setting lexer properties (FIXME).
* Labels and comments are currently styled the same.
  This could change in the future once we introduce real comments.
* Lexers are usually implemented in C++, but I did not want to draw in C++.
  Especially not since we'd have to include parser.h and other SciTECO headers,
  that really do not want to keep C++-compatible.
  Instead, the lexer is implemented "in the container".
  @ES/SCI_SETILEXER/sciteco/ is internally translated to SCI_SETILEXER(NULL)
  and we get Scintilla notifications when styling the view becomes necessary.
  This is then centrally forwarded to the teco_lexer_style() which
  uses the ordinary teco_view_ssm() API for styling.
* Once the command line becomes a Scintilla view even on Curses,
  we can enabled syntax highlighting of the command line macro.

* g_assert() apparently does not reference the expression when assertions are disabled
  in contrast to glibc's assert()

* Has been observed on PDCursesMod/WinGUI when pressing CTRL+Shift+6 on an US layout.
  I would expect code 30 (^^) to be inserted, instead PDCurses reports two keypresses (6^^).
  The first one is now filtered out since this will not be fixed upstream.
  See also https://github.com/Bill-Gray/PDCursesMod/issues/323
* Since AltGr on German layouts is reported as CTRL+ALT, we must be careful not to filter those
  out as well.
* This is active on all PDCurses variants - who knows which other platforms will behave similarily.
* You still cannot insert code 0 via CTRL+@ since PDCurses doesn't report it, but ncurses does not allow that either.
  This _could_ be synthesized by evaluating the modifier flags, though.

* Fixes the test suite on PDcurses/Win32 and therefore CI builds.
* Should be necessary on UNIX as well since later on, we would access
  cmdline_window, which is not yet initialized.
  I didn't see any errors in Valgrind, though.

* Apparently, netbsd-curses overwrites the escdelay on initscr() (if $ESCDELAY is not set),
  so we have to apply the default 25ms after screen initialization.
* The info line is not drawn correctly on netbsd-curses, but only on st/simpleterm.
  I assume this is just a shortcoming of the included terminfo entry.

* added TECO_ERROR_CLIPBOARD for all clipboard-related errors

* Curses: "icons" have also been added