Moves ui references from the tgui subsystem to datums themselves (#76215)

## About The Pull Request

Unused vars have 0 memory cost, and the ref and list lookup here is
REALLY expensive, for both init and foam spreading.

## Why It's Good For The Game

Saves 0.2s off a station flood on meta, and 0.17s off init. More time in
other qdel heavy areas
Pulled off #76104 for the sake of cleanliness

code/datums/datum.dm

@@ -17,6 +17,10 @@
 	  */
 	var/gc_destroyed
 
+	/// Open uis owned by this datum
+	/// Lazy, since this case is semi rare
+	var/list/open_uis
+
 	/// Active timers with this datum as the target
 	var/list/_active_timers
 	/// Status traits attached to this datum. associative list of the form: list(trait name (string) = list(source1, source2, source3,...))