fix up fit-selected-tracks so that we are filling the trackview area, not the entire canvas (which now includes rulers)
rename Editor::_canvas_bottom_rect as Editor::_canvas_drop_zone; increase its size; remove debug message
change height of canvas drop so last 20 pixels of bottom track is still visible when fully scrolled down
forward scroll events from track control headers to the canvas, to get consistent scroll behaviour (whatever that behaviour is)