Allow applications to select to which track a track object should be added

Allow applications to select to which track a track object should be added

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