for the undo history, so that changes to regions' layering_index
get stored in the undo record. Make Playlist::update use add_region_internal
so that undone regions don't have their layering_index corrupted.
Setup layering indices on relayer() so that deletion of regions
causes an update.
git-svn-id: svn://localhost/ardour2/branches/3.0@11123 d708f5d6-7413-0410-9779-e7cbd77b26cf