When a track is in "stacked" mode, allow regions to be moved to different layers...

When a track is in "stacked" mode, allow regions to be moved to different layers by dragging
and dropping.  In stacked mode, the relayering by "recently added / moved etc. is higher" setting
is ignored, and regions are put on the layer that they are dropped to, unless:

1. they are dropped so that they overlap another region on the same layer, in which case they
are put on the layer above that one.
2. they are dropped such that they could be put on a lower layer with no audible difference, in
which case they are put on the lower layer.

git-svn-id: svn://localhost/ardour2/branches/3.0@5590 d708f5d6-7413-0410-9779-e7cbd77b26cf