Canvas ND: keep canvases through reinit

Canvas ND: keep canvases through reinit

Make fdm listener single-fire, don't listen to /sim/signals/reinit. This
allows the Canvas to stay with the same placement through reinit, after
both the 777 and 747 were having problems. I don't see any reason for
having to recreate it all, and the cleanup function is still there (e.g.
for independent windows, to have their .del() call the ND's .del()).

renamed handle_reinit() -> del()