Jingle: Replace X11 specifics with GstGtkSink

This should eventually allow video chat to work on non-X11 systems, such as ones using Wayland.

Here is the Gstreamer graph generated on my computer when calling someone:

Note that the Jingle negotiation still ends before the stream is established, for some reason.

Fixes #9515 (closed).

Fixes #9832 (closed).

Maybe improves things for #6030 (closed) too.