* You couldn't see the first character on a new line after wrapping
  at the end of a multiline command line.
* This is only worked around. The real reason must be in Scintilla,
  perhaps in a faulty calculation in Editor::MaxScrollPos().

* teco_string_diff() could return a number of bytes in the middle of
  an Unicode sequence. It now also requires Unicode strings.
* Added a missing Unicode-validity check when replacing command lines (`{` and `}`).
  teco_cmdline_insert() should really be refactored, though (FIXME).
* Added test case

* When passing a struct that should not be modified, I usually use a const pointer.
* Strings however are small 2-word objects and they are often now already passed via separate
  `gchar*` and gsize parameters. So it is consistent to pass teco_string_t by value as well.
  A teco_string_t will usually fit into registers just like a pointer.
* It's now obvious which function just _uses_ and which function _modifies_ a string.
  There is also no chance to pass a NULL pointer to those functions.

* Requiring state callbacks by generating their names (e.g. NAME##_input) has several disadvantages:
  * The callback is not explicitly referenced when the state is defined.
    So an unintroduced reader will see some static function, which is nowhere referenced and still
    doesn't cause "unused" warnings.
  * You cannot choose the name of function that implements the callback freely.
  * In "substates" you need to generate a callback function if you want to provide a default.
    You also need to provide dummy wrapper functions whenever you want to reuse some existing
    function as the implementation.
* Instead, we are now using static assertions to check whether certain callbacks have been
  implemented.
  Unfortunately, this does not work on all compilers. In particular GCC won't consider
  references to state objects fully constant (even though they are) and does not allow
  them in _Static_assert (G_STATIC_ASSERT). This could only be made to work in newer GCC
  with -std=c2x or -std=gnu23 in combination with constexpr.
  It does work on Clang, though.
  So I introduced TECO_ASSERT_SAFE() which also passes if the expression is *not* constant.
  These static assertions are not crucial - they do not check anything that can differ between
  systems. So we can always rely on the checks performed by FreeBSD CI for instance.
  Also, you will of course quickly notice missing callbacks at runtime - with and without
  additional runtime assertions.
* All mandatory callbacks must still be explicitly initialized in the TECO_DEFINE_STATE calls.
* After getting rid of generated callback implementations, the TECO_DEFINE_STATE macros
  can finally be qualified with `static`.
* The TECO_DECLARE_STATE() macro has been removed. It no longer abstracts anything
  and cannot be used to declare static teco_state_t anyway.
  Also TECO_DEFINE_UNDO_CALL() also doesn't have a DECLARE counterpart.

We cannot rely on the central teco_cmdline_update() call per keypress
in this case.
The analogous call was removed in 4e6ddd6c329d56055a732c6344df019f0d997aaf,
so this was a recent regression.

* The line end type and tab mode is apparently a property of the document.
  Therefore it was lost when exchanging the command line's document during command line replacement.
* Instead, the old command line is now stored in a string.
* During replacement we delete and append only the part of the command line that changed.
  This ensures that we don't have to restyle the entire command line with every single
  replacement (even if it is at the end of the command line).

* This allows for several customizations.
  * You can simply increase the visible command line history.
    For that you must also set SCI_SETWRAPMODE(SC_WRAP_CHAR).
    An example was added to fallback.teco_ini.
  * You could also set SCI_SETLINEENDTYPESALLOWED(SC_LINE_END_TYPE_DEFAULT)
    to see the structure of inserted text.
* Alternatively we could have introduced a new command like EP or FW
  and also overload it to replace the current ED&2048 (e.g. -EP and EP).
  In DEC TECO `W` comes closest to what 5EJ now does.