fix text placement in ArdourCanvas::Flag
passthrough unhandled motion events in MIDI region views because we may be dragging non-note objects like patch changes
delete canvas Flag object when deleting a PatchChange, and provide a canvas debug name for PatchChanges
when delivering canvas events, if an item is grabbed, use it in preference to Canvas::_current_item, but still propagate to its ancestors if left unhandled
fix major thinko in ArdourCanvas::Group::clear()