GitLab

Requires libxfce4ui >=4.13.2

The thumbnailer code is a bit hairy, but it appears to work.
Let's wait for the crashes and bug reports.

Optional plugins are not ported yet. We use a shitload of
deprecated Gtk APIs now. Not all features work.
It doesn't look great yet. There are bugs.

But, well, it's a first step.

Building it fails starting with gtk-doc-1.22, and there have been
lots of validity errors with previous versions for some time. Until
someone comes and fixes it, stop building it.

This functionality is now provided by libxfce4ui, see commit
8990efaf "Use the spawn code from libxfce4ui."

Fix warning about m4 directory.

See https://bugzilla.xfce.org/show_bug.cgi?id=8930 for details.
Signed-off-by: Simon Steinbeiss <simon.steinbeiss@elfenbeinturm.at>