Fix to stacked layering for the case when a region is dragged to overlap another...

Fix to stacked layering for the case when a region is dragged to overlap another.  Remove unused signal LayeringChanged from Playlist.  Cleanups in AudioRegion, and ensure that region envelopes cannot become empty when regions are split (and maybe in some other cases too).  Empty envelopes cause crashes when e.g. AutomationList::truncate_at_start() is called.

git-svn-id: svn://localhost/ardour2/trunk@2513 d708f5d6-7413-0410-9779-e7cbd77b26cf