ges-track: Add the notion of resriction caps to GESTrack

This way we can let the user determine what he want to come out of the
track.

API:
  - ges_track_set_caps (The track caps are now construct only)
  + ges_track_set_restriction_caps
  + ges_track_get_restriction_caps
  + GESTrack.props.restriction_caps
gstframepositionner: Install width and height properties.

  + And manage them properly.
videotransition: No need to hard set width and height anymore.
test-utils: Adds a utility function to quickly check the timeline.
tests: timelineedition: Add a test_scaling.

It will check that the clip updates its size correctly.
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
gstframepositionner: correctly tag metadata.

We do not use GST_VIDEO_META_TAG_STR as it would mean depending on
GstVideo which is not the case right now
ges: Make GESAudioSource and GESVideoSource abstract