ges: Make GESTimeline responsible for adding GESTrackElement to GESTrack

ges: Make GESTimeline responsible for adding GESTrackElement to GESTrack

	+ Fix tests as necessary (Do not use agingtv as it can be "applied" on any TrackType
    	   and is not representative of what happens IRL)

We already had the infrastructure so the user can have the control over where to add
the elements (through the "select-track-for-object" signal). We now make use of that
signal everytime a GESClip is added to a GESTimelineLayer. This make user's life easier,
and object responsability clearer.