tree/container: remove event listeners on destroy

Change begin_destroy to remove event listeners before the final destroy,
since otherwise event listeners would be removed twice, which crashes.

This fixes a crash in wlroots listener checks. See #8509.

include/sway/tree/container.h

@@ -95,6 +95,7 @@ struct sway_container {
 
 	struct wl_listener output_enter;
 	struct wl_listener output_leave;
+	struct wl_listener output_handler_destroy;
 
 	struct sway_container_state current;
 	struct sway_container_state pending;