fix crash caused by click-on-region-name if name highlights are not in use - such a click is not a trim operation under those conditions
add Item::canvas_origin() for convenience
when delivering enter/leave events to canvas items, ensure that the event coordinates are in canvas space, not window space

This fixes a variety of borkage in the canvas at present
introduce Drag::_trackview_only which indicates that all y-axis positions for the pointer during a drag should be relative to the top of the trackview group.

More or less all drags except for Cursor (playhead) and drags in the range marker bars have this true (which is the default value)