part one of allowing region drag to the bottom (drop-zone) rect of track canvas
remove unused variable
change return type of Editor::axis_view_from_route()
non-copy region drag now creates a new track when a region is dragged to the drop zone
adding a new track via copy-region-drag now works (and shares code with the non-copy case
comment tweak
remove unnecessary and wierd constant for bottom rect (drop zone) of track canvas