* 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.

* M[lexer.set.cmdline] can be used to set up syntax highlighting on the command line
  (if desired).
* Color schemes with light-dark themes (solarized.tes) are now responsible
  to update the command line view as well.

* Practically all calls to teco_expressions_args() must be preceded by teco_expressions_eval().
* In code paths where we know that teco_expressions_args() > 0, it is safe
  to call teco_expressions_pop_num(0) instead of teco_expressions_pop_num_calc().
  This is both easier and faster.
* teco_expressions_pop_num_calc() is for simple applications where you just want to get
  a command argument with default (implied) values.
  Since it includes teco_expressions_eval(), we can avoid superfluous calls.
* -EC...$ turned out to be broken and is fixed now.
  A test case has been added.

* It turns out that `bool` (_Bool) in bitfields may cause
  padding to the next 32-bit word.
  This was only observed on MinGW.
  I am not entirely sure why, although the C standard does
  not guarantee much with regard to bitfield memory layout
  and there are 64-bit available due to passing anyway.
  Actually, they could also be layed out in a different order.
* I am now consistently using guint instead of `bool` in bitfields
  to prevent any potential surprises.
* The way that guint was aliased with bitfield structs
  for undoing teco_machine_main_t and teco_machine_qregspec_t flags
  was therefore insecure.
  It was not guaranteed that the __flags field really "captures"
  all of the bit field.
  Even with `guint v : 1` fields, this was not guaranteed.
  We would have required a static assertion for robustness.
  Alternatively, we could have declared a `gsize __flags` variable
  as well. This __should__ be safe since gsize should always be
  pointer sized and correspond to the platform's alignment.
  However, it's also not 100% guaranteed.
  Using classic ANSI C enums with bit operations to encode multiple
  fields and flags into a single integer also doesn't look very
  attractive.
* Instead, we now define scalar types with their own teco_undo_push()
  shortcuts for the bitfield structs.
  This is in one way simpler and much more robust, but on the other
  hand complicates access to the flag variables.
* It's a good question whether a `struct __attribute__((packed))` bitfield
  with guint fields would be a reliable replacement for flag enums, that
  are communicated with the "outside" (TECO) world.
  I am not going to risk it until GCC gives any guarantees, though.
  For the time being, bitfields are only used internally where
  the concrete memory layout (bit positions) is not crucial.
* This fixes the test suite and therefore probably CI and nightly
  builds on Windows.
* Test case: Rub out `@I//` or `@Xq` until before the `@`.
  The parser doesn't know that `@` is still set and allows
  all sorts of commands where `@` should be forbidden.
* It's unknown how long this has been broken on Windows - quite
  possibly since v2.0.

* Being able to embed null bytes into the lParam string is
  practically useless - there aren't any messages where this is useful
  and where there are no native SciTECO counterparts - so this case is now catched
  and the null-byte separates wParam from lParam.
* wParam can be the empty string, but it is not supported to pass wParam as a
  string and lParam as the empty string.
  If the second string argument ends in ^@, lParam is popped from the stack instead.
* This is a temporary workaround until we can properly parse the Scintilla.iface and
  generate more elegant per-message wrappers.
* It in particular unlocks the SCI_SETREPRESENTATION and SCI_SETPROPERTY messages.
  The former allows us to write a special hex-editor macro which sets hexadecimal
  character representations, while the latter allows you to set lexer properties.
* The C-based lexers ("cpp" in Lexilla) can now take preprocessor definitions into account.
  This is disabled by default, unless you set lexer.c.defines before opening a file.
  You can also set it interactively and re-set the lexer. For instance:
  ^U[lexer.c.defines]NDEBUG$ M[lexer.set.c]

* The "own" objects are tricky to work with and have special requirements,
  so try to avoid them.
* Also, wrap the push functions in macros like all other scalars.
* This is a purely cosmetic change, but avoids some confusion.

* Curses allows scrolling with the scroll wheel at least
  if mouse support is enabled via ED flags.
  Gtk always supported that.
* Allow clicking on popup entries to fully autocomplete them.
  Since this behavior - just like auto completions - is parser state-dependant,
  I introduced a new state method (insert_completion_cb).
  All the implementations are currently in cmdline.c since there is some overlap
  with the process_edit_cmd_cb implementations.
* Fixed pressing undefined function keys while showing the popup.
  The popup area is no longer redrawn/replaced with the Scintilla view.
  Instead, continue to show the popup.