Add a video preview widget in the preferences

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).