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
In TimelineTitleSource We do not add a TrackAudioTestSource in the audio track
anymore as it was a hack to work around the fact that we used not to have gap
support, now we do, remove related tests
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