Make popdown,popup methods idempotent; remove 'popdown' for 'cancelled'

Make popdown,popup methods idempotent; remove 'popdown' for 'cancelled'

Callers will generally expect _popup and _popdown to be a no-op if
the menu is already in that state; make it so.

Also change the 'popdown' signal to be 'cancelled'; this is
clearer and allows us to avoid having activate also call popdown.

https://bugzilla.gnome.org/show_bug.cgi?id=594699