Video: Added a new GmVideoWidget.

This widget is able to display two video streams, a primary and a
secondary one. The secondary one is displayed in a PIP window when both
streams are active.

Several interesting parameters are defined as properties, which allow
the use of GSettings bindings.
VideoOutputManager: Added convenience enum for MAX.
Call Window: Use the new GmVideoWidget.
GmVideoWidget: Restore the logo to its original size on stream close.
CallWindow: Signal the GmVideoWidget that the streams are closed.
ClutterGst: Fixed open signal propagation.
Video: Implemented extended video output with GmVideoWidget.

This is now a GmWindow.