* The GTK version logs additional warnings, so we cannot
  match verbatim against stderr.
  Instead we only look for a line beginning with `Warning:` or `Error:`.
* We now also test info messages (`1^A`).

* I.e. you can now log warnings and errors from SciTECO code as well.
* We do not need a version of ^A accepting code points, since this is
  supported by ^T already.

In TECO-11 pattern matching constructs do not allow backtracking,
while the PCREs currently generated do allow backtracking.
This would be easy to fix, but there should also be constructs
to re-enable the backtracking semantics.
I left only a Known Bug test case for the time being.

* 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

It causes build failures on OBS for several Linux distributions.
It generally works on Linux, so it's probably related to Xvfb.

Hopefully fixes the OBS build errors.

This is useful to launch from a terminal without "blocking" this terminal.
There are tools like nohup and daemonize (BSD) to do the same, but having it
builtin is shorter to write.

It was returning the range of the search, but not of the inserted text.
Since the searched text is deleted, the range of the insertion is more useful.
It's also what was documented and what DEC TECO does.