[VideoSinkGStreamer] Lower video frame callback priority

To avoid the frame callback being starved by other sources it was set
to run with rather high priority, G_PRIORITY_DEFAULT.

Lowering it to G_PRIORITY_DEFAULT_IDLE allows for GDK_PRIORITY_REDRAW
(ie. G_PRIORITY_HIGH_IDLE+20) to be run even while a video is playing,
restoring window resizing behavior and redrawing of chrome widgets
in applications.

Using G_PRIORITY_DEFAULT_IDLE seems to also fix the timeout that
hides the video controls when the video is no longer hovered by the
mouse pointer.