integration tests: adds a test for scaling video.
videotransition: No need to hard set width and height anymore.
Install width and height properties in ges-video-source.
framepositionner: put width and height handling there.
ges-track: Remove the set_caps from the API, keep it internally.

	   + Add restriction caps instead.
	   + gnonlin sets these caps on all its children, which
	     is not really convenient.