* Instead of rendering a teco_string_t into a Scintilla view (GTK)
  and an ncurses window (Curses), it is now a Scintilla view and document
  that is modified directly.
* Reduces redundancies between GTK and Curses UIs.
* It will be more efficient on very large command lines, especially on GTK.
* We can now redirect Scintilla messages to the command line view in order
  to configure syntax highlighting, the margin, rubout indicator style and
  scroll behavior (TODO).
* This will also simplify the configuration of multi-line command lines (TODO).
* Since INDIC_PLAIN is not supported by Scinterm, rubbed out command lines
  are now styled with INDIC_STRAIGHTBOX (background color).

* The teco_qreg_vtable_t::get_string() method should support returning the
  length optionally (may be NULL).
  This already worked with teco_doc_get_string(), even though it wasn't documented,
  and therefore didn't cause problems with regular Q-Registers.

* When enabled with bit 2 in the ED flags (0,4ED),
  all registers and buffers will get the raw ANSI encoding (as if 0EE had been
  called on them).
  You can still manually change the encoding, eg. by calling 65001EE afterwards.
* Also the ANSI mode sets up character representations for all bytes >= 0x80.
  This is currently done only depending on the ED flag, not when setting 0EE.
* Since setting 16,4ED for 8-bit clean editing in a macro can be tricky -
  the default unnamed buffer will still be at UTF-8 and at least a bunch
  of environment registers as well - we added the command line option
  `--8bit` (short `-8`) which configures the ED flags very early on.
  As another advantage you can mung the profile in 8-bit mode as well
  when using SciTECO as a sort of interactive hex editor.
* Disable UTF-8 checks in 8-bit clean mode (sample.teco_ini).

* ^Uq however always sets an UTF8 register as the source
  is supposed to be a SciTECO macro which is always UTF-8.
* :^Uq preserves the register's encoding
* teco_doc_set_string() now also sets the encoding
* instead of trying to restore the encoding in teco_doc_undo_set_string(),
  we now swap out the document in a teco_doc_t and pass it to an undo token.
* The get_codepage() Q-Reg method has been removed as the same
  can now be done with teco_doc_get_string() and the get_string() method.

* I decoded the Scintilla charset values into codepages, at least
  those used on Gtk.
* make sure that the line character index is not allocated or released
  too often, as it is actually internally reference counted, which
  could result in it missing when we really need it.
* The line character index still appears to be released whenever
  the document pointer changes, which will happen after using
  a different Q-Register.
  This could be a performance bottleneck (FIXME).

* ^E is heavily overloaded and can also be used to check whether a given index is valid
  (as it is the same that most movement commands to internally).
  Besides that, it is mainly useful for interfacing with Scintilla messages.
* EE takes a code page or 0 for ANSI/ASCII.
  Currently all documents and new registers are UTF-8.
  There will have to be some kind of codepage inheritance and a single-byte-only mode.

* this required adding several Q-Register vtable methods
* it should still be investigated whether the repeated calling of
  SCI_ALLOCATELINECHARACTERINDEX causes any overhead.