layer_shell: order destroying before sway_output

Both sway_output and sway_layer_shell listen to wlr's output destroy event,
but sway_layer_shell needs to access into sway_output's data strucure and needs
to be destroyed first.

Resolve this by making sway_layer_shell listen to a new event that happens at
start of sway_output's destroy handler

include/sway/output.h

@@ -32,6 +32,10 @@ struct sway_output {
 	struct wl_list link;
 
 	pid_t bg_pid;
+
+	struct {
+		struct wl_signal destroy;
+	} events;
 };
 
 void output_damage_whole(struct sway_output *output);