Comment and (hopefully) simplify Editor::region_drag_motion_callback, fixing problems...

Comment and (hopefully) simplify Editor::region_drag_motion_callback, fixing problems with dragging regions
to and from tracks in "stacked" display mode.  As a side-effect, enable dragging of regions between layers,
though I'm not entirely sure where this is useful or desirable just yet; dragging regions between layers
currently works in the GUI but has no effect on the model.

git-svn-id: svn://localhost/ardour2/branches/3.0@4389 d708f5d6-7413-0410-9779-e7cbd77b26cf