Fix crash when deleting overlapped regions.

Fix crash when deleting overlapped regions.

Use RegionSelection for MIDI regions as well, since the old dumb stub didn't do
some things correctly.  There's probably no reason to have a separate class for
this at all, and some good ones for putting all regions in the same selection,
so we should probably do that.  For now they are still separate in the
selection but use the same base class.