Remove GESTrackElements from GESTracks when removing from a GESClip

Remove GESTrackElements from GESTracks when removing from a GESClip

... Not the other way round.

  + Add and enhance debugging info on the way

The user should not be responsible for removing the GESTrackElements from
GESTracks, instead, removing it from a GESClip should imply removing
it from any GESTrack it is in.

This patch changes sensibly the behaviour when we remove a
GESTrackElement from a GESTrack, not remoing it from the GESClip it is
in. *But*, users should never remove a GESTrackElement from a GESTrack
anyway. The testsuite has been updated to that new behaviour.