Remove APIs:
ges_track_element_set_locked
ges_track_element_is_locked
Those APIs where really not nice to use and were causing more issues
than solving them. If 2 time related properties of TimelineElement must
be different, then those element can *not* have the same parent.
Plus, with the new ges_container_group () API, we will recreate 1
GESClip containing the proper GESTimelineElements if it is the thing
to do.