Jingle video: Replace X11 specifics with GTK+

This uses the same mechanism originally added for preferences, so that
video display is handled by a GTK+ widget instead of some X11 dark
magic.

This makes the self video display properly (!) during a Jingle call, but
still with random freezes and no remote video being displayed, those
fixes will be for a latter commit.
3 jobs for jingle in 2 minutes and 47 seconds (queued for 2 minutes and 48 seconds)
Status Job ID Name Coverage
  Test
passed #14692
run-appdata

00:00:04

passed #14691
run-tests

00:02:34

 
  Build
passed #14693
run-build

00:00:09