Be careful about the list of event controllers

Be careful about the list of event controllers

We can end up with _gtk_widget_remove_controller getting called
while we are iterating over the list in _gtk_widget_run_controllers.
To avoid trouble, only mark the event controller as dead by
setting data->controller to NULL, and defer the actual freeing
and list manipulation to the loop in _gtk_widget_run_controllers.
Update other places that operate on controllers to handle
data->controller being NULL.