desktop/output: Debounce modesets

Output changes often happen in rapid succession. Instead of doing the
modesets one by one, set a 10 millisecond debounce timer.

include/sway/server.h

@@ -133,6 +133,8 @@ struct sway_server {
 	// Stores the nodes that have been marked as "dirty" and will be put into
 	// the pending transaction.
 	list_t *dirty_nodes;
+
+	struct wl_event_source *delayed_modeset;
 };
 
 extern struct sway_server server;