widget: Free the controller list on finalize()

widget: Free the controller list on finalize()

If this is done on dispose(), the widget may be destroyed (and its
controllers list NULLified) within _gtk_widget_run_controllers(),
causing warnings/crashes when it just tried to hop on the next
controllers.

Freeing the controllers here should be a safety net for implementations,
so it also makes sense to do this late. The widgets that choose to
free their controllers on dispose can still do so, and get
_gtk_widget_remove_controller() called for these as an indirect result.