Modifies some API:
ges_timeline_object_create_track_objects now take a GESTrackType instead of a
GESTrack as second argument, and return a GList instead of a boolean
ges_timeline_object_create_track_object now take a GESTrackType instead of a
GESTrack as second argument
API: GESTrackObjectClass::gnlobject_factorytype
API: GESTrackObjectClass::create_element
Most track objects are only specific by the contents of the gnlobject,
therefore move the 'create_element' vmethod which was already present
in some subclasses to the top-level class.
Also make the code more robust
