track-element: change owner to creator

track-element: change owner to creator

Rename the private "owners" to "creators" to avoid confusing this with
the owner of the track element's memory.

Also made the ungroup method for GESClip symmetric by making all the
children of the resulting clips share their creators, which allows them
to be added to any of the other ungrouped clips. Once the clips are
grouped back together, the tracks loose these extra creators.