vv: Switch to GTK sinks by default.

Set minimum gstreamer to 1.14.

Stop trying to set window id on GStreamer sinks. This is all handled by gtksink.

Use gtksink's embedding widget in conversations. This replaces the previous window id method, and doesn't need fiddling with GStreamer events.

Testing Done:
Compiled; tested prefs window. Did not check conversations because I don't know how to test that.

Reviewed at https://reviews.imfreedom.org/r/154/