fix an old bug introduced when cth tried (valiantly) to make region-list -> DnD -> canvas behave like a regular region drag. this doesnt' work because the RegionInsertDrag() grabs the mouse and breaks the DnD. in addition, the code failed to correctly indicate when a drop was possible (over a Track) and when it was not (over a Bus) - this has also been fixed. the actual code in Editor::drop_regions() could and should be simplified - no reason to use a RegionInsertDrag here, but it was fast