+ Fix unit tests
  + Minor enhancement in unit tests

API changes:
-----------
  * ges_track_element_get_clip     -> ges_timeline_element_get_parent
  * ges_clip_add_track_element     -> ges_container_add
  * ges_clip_release_track_element -> ges_container_remove
  * ges_clip_get_track_elements    -> ges_container_get_children
                                      (or GES_CONTAINER_CHILDREN)