* 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.

* This is still rendered as underlined text.
* It uses a new Scinterm upstream feature, so we can
  switch back to the upstream repo.

* 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.

* Other editors call "backup files" previous copies of saved files.
  This role would be served by savepoint files in SciTECO.
* Likewise filename~ would point to such a backup file.
  It therefore makes sense that savepoint files also end in tildes (.teco-n-filename~).
* Security copies of modified buffers would be called "auto-saves" (Emacs) or
  "swap files" (Vim).
  Both of these terms is IMHO misleading, so SciTECO now uses the
  term "recovery file".
* "Recovery files" are now named #filename# just like in Emacs.

* The backup mechanism is supposed to guard against crashes of SciTECO and
  unexpected program terminations (e.g. power cycling, etc.)
* In a given interval (no matter whether busy or idlying on the prompt)
  SciTECO saves all modified buffers with the filename~ (like most other editors).
  As an optimization files are not backed up if they have been backed up
  previously to avoid pointless and possibly slow file system writes.
* While the backup mechanism exists outside of the usual undo-paradigm -
  backup file creating is not bound to character input and it makes no sense
  to restore the exact state of backup files - there are some interesting
  interactions:
  * When a buffer is dirtyfied or saved that was previously backed up, it must always
    be reset to the DIRTY state on rubout, so backups are eventually recreated.
  * When a buffer is dirtyfied first (was clean), the backup file must be
    removed on rubout as well - we don't expect backup files for clean buffers.
* There is currently no automatic way to restore backup files.
  This could potentially be done by opener.tes and session.tes in the future,
  although you couldn't currently always get meaningful user feedback
  (whether he wants to restore the file).
  Perhaps we should at least log a message when detecting backup files that
  are newer than the file that is being opened.

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.

0x404040 (COLOR_LBLACK) is a poor choice since on terminal emulators
with less than 16 colors, it would be rendered in black (and thus be invisible).

This requires enhanced INDIC_STRAIGHTBOX semantics, which are not yet upstream in Scinterm.