Makes events global.

Greatly decreases the number of objects that need to be created (currently 3 event handlers in total vs basically 3 per datum)
The price is a slower lookup of listeners but on a per round basis it should still be much less than the current startup time penalty.

code/unit_tests/observation_tests.dm

@@ -1,5 +1,5 @@
 /proc/is_listening_to_movement(var/atom/movable/listening_to, var/listener)
-	return listening_to.moved.is_listening(listener)
+	return moved_event.is_listening(listening_to, listener)
 
 datum/unit_test/observation
 	name = "OBSERVATION template"