* An empty message line would actually contain "(Unnamed)".
* Info popups must discern the "(Unnamed)" string (e.g. a file with that name)
  from the actual unnamed buffer since when clicking the unnamed buffer, you
  must insert nothing (`EB$`). Therefore, the unnamed buffer is represented
  as an empty string. "(Unnamed)" is just a placeholder for rendering.
  This was carried over into TecoGtkLabel which always rendered the empty string
  as "(Unnamed)".
  But TecoGtkLabels are used for the info and message lines as well.
* Therefore the fallback/placeholder string is now configurable per label.
* On the downside, this wastes one more machine word per TecoGtkLabel.
  The alternative would have been to use {NULL, 0} as the representation
  for unnamed buffers, so you can actually discern the empty string from the
  unnamed buffer representation.
  However it feels wrong to have this kind of info-popup-specific handling
  in a more generic label widget.
  Also, for consistency we'd have to touch the Curses UI as well where
  the unnamed buffer is also currently internally represented by empty
  strings (as opposed to NULL).
* Regression introduced by 0c89fb700957e411885e7e7835e15f441e8b5e84,
  so it was in v2.5.0.

* 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.

* When constructing the list of popup items, the unnamed buffer is stored as the empty string
  instead of a prerendered "(Unnamed)".
  Using the empty string simplifies autocompletions, which will actually have to insert nothing
  at all (in addition to terminating the string).
* Since unnamed buffers are now special in the popup list, we can render them with special
  icons as well.
  Currently, only on Curses we use a file symbol with a question mark.
  There doesn't appear to be a fitting standard Freedesktop icon to use on GTK and there
  isn't even any fitting standard emblem to lay over the default file icon.

* Gtk 3.24 has been introduced accidentally in 9e3746a4
  due to GtkEventControllerScroll.
* It would be possible to still support v3.12 by partially
  reversing 9e3746a4 and conditionally including teco_interface_scroll_cb().
  But it's probably not worth the trouble.

* n:^T always prints bytes (cf. :^A)
* ^T without arguments returns a codepoint or byte from stdin.
  In interactive mode, this currentply places a cursor in the message line and waits for a keypress.

* 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.

* This probably did not cause any bugs.