Preferences: Add a video preview widget

In order to work on every system Gajim runs on (instead of only X11),
this new widget uses GTK+ Gstreamer elements.

It currently completely breaks the actual Jingle thing, but that was
already pretty broken so will be fixed in a later commit.

It might be nice to think about the way we want to make preferences
interact with a currently-running audio/video session, since it
currently uses a separate pipeline and thus will conflict when e.g. both
will want to access the same camera device, if the OS doesn’t implement
multiplexing (Linux doesn’t).
3 jobs for av-preferences-preview in 3 minutes and 7 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #12756
run-appdata

00:00:08

passed #12755
run-tests

00:02:48

 
  Build
passed #12757
run-build

00:00:11