output: emit node::destroy event

Now output_begin_destroy emits the node::destroy event similar to
workspace_begin_destroy. It currently has no listeners, since they
listen to output::disable or wlr_output::destroy instead.

sway/tree/output.c

@@ -286,6 +286,7 @@ void output_begin_destroy(struct sway_output *output) {
 		return;
 	}
 	sway_log(SWAY_DEBUG, "Destroying output '%s'", output->wlr_output->name);
+	wl_signal_emit(&output->node.events.destroy, &output->node);
 
 	output->node.destroying = true;
 	node_set_dirty(&output->node);