Some cleanup in the GStreamer plugin.

Removed QGstreamerVideoOverlay and related classes. It was used as
'Window' control but performs worse than QGstreamerVideoWindow which
does basically the same thing using GStreamer ready-made components
instead.
Removed X11 dependencies and related configuration tests. It was only
needed for QGstreamerVideoOverlay.

Change-Id: I2ad2636ccf0060e56cd64f3d9e5b3c24dc75f5a3
Reviewed-by: Andy Nichols <andy.nichols@digia.com>