* this was noticable only after the first character, i.e. after `#x`.
* Q-register tables are case sensitive, so we must not call
  teco_rb3str_auto_complete() with case_sensitive == FALSE.
  And it's also entirely unnecesary since the Q-register name
  for two-character Q-registers is already case-folded.

* 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.

* This has long been a TECO-11 incompatibility.
* The first label in a list has index 0, i.e. `1Ofoo,bar$` jumps to label `!bar!`.
  Consequently 0 is also implied, so `Olabel$` continues to do what you expect.
* `0Ofoo$` was previously also jumping to `!foo!` which was inconsistent:
  All invalid indexes should do nothing, i.e. execution continues after the go-to command.
* Fixed handling of empty labels as in `1Ofoo,,bar$` - execution should also continue
  after the command.
  This eases writing "default" clauses immediately after the go-to.
* The ED hook values now also begin at 0, so most existing ED hook macros should
  continue to work.
* Similarily, the mouse events returned by -EJ also begin at 0 now,
  so fnkeys.tes continues to work as expected.
* It's still very possible of course that this breaks existing code.

Works via a default implementation in the "external" Q-register "class"
by first querying the string, appending and re-setting it.

* `static const char *p = "FOO"` is not a true constant since
  the variable p can still be changed.
  It has to be declared as `static const char *const p = "FOO"`,
  so that the pointer itself is constant.
* In case of string constants, it's easier however to use `static const char p[] = "FOO"`.

* `[q]~` was broken and resulted in crashes since it reset the clipboard character to 0.
  In fact, if we don't want to break the `[a]b` idiom we cannot use the numeric cell
  of register `~`.
* Therefore we no longer use the numeric part of register `~`.
  Once the clipboard registers are initialized they completely replace
  any existing register with the same name that may have been
  set in the profile.
  So we still don't leak any memory.
  (But perhaps it would now be better to fail with an error
  if one of the clipboard registers already exist?)
* Instead, bit 10 (1024) of ED is now used to change the default
  clipboard to the primary selection.
  The alternative might have been an EJ flag or even a special register containing
  the name of the default clipboard register.
* partially reverses 8c6de6cc718debf44f6056a4c34c4fbb13bc5020

* This command exists in Video TECO.
  In Video TECO it also supports reading multiple files with a glob pattern -- we do not support that
  as I am not convinced of its usefulness.
* teco_view_load() has been extended, so it can read into dot without
  discarding the existing document.