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
NOTE: The caller of ges_timeline_object_create_track_object now needs to add it to
the timeline_object after calling the function.
Fix the testsuite to support that accordingly
Allows moving independently (or not) timelineobjects and trackobjects and
have them synchronized with the offsets taken into account.
Right now only the start and priority properties are synchronized. The duration
and in-point properties will require more thoughts.