GES: Add video and audio base sources.

    + Update documentation.
    + Implements subclasses audio-uri-source and video-uri-source
GES: Port all sources to inheriting from audio / video base sources.

That was painful.
videotransition: No need to hard set width and height anymore.
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.
gstframepositionner: Install width and height properties.

    + And manage them properly.
test-utils: Adds a utility function to quicly check the timeline.

    + It takes a dump_to_dot argument as a free bonus.
timelineedition test: Adds a test_scaling.

    + It will check that the clip updates its size correctly.
    + Will certainly move elsewhere.
timeline-element: Add a set_parent vmethod

API:
        GESTimelineElment->set_parent vmethod
videosource: Make sure to update z-order when layer priority changes

Conflicts:
	ges/ges-video-source.c
FIXUP vmethod
Misc fixes for class hierarchy rework FIXUP
gstframepositionner: correctly tag metadata.