Datum var cleanup (#34314)

* Consolidates datum var location

* Move ui_screen to /datum/tgui

* Move focusers to datum.dm. Move fingerprintslast to /atom

* Remove focusers list, makes mobs check if their focus is QDELETED in Life()

* Consolidate use_tag and var_edited into datum_flags

* Revert garbage comment

* Thought I already removed this

* Remove the var_edited preservation

* Removes focus QDELETED handling

code/modules/mob/mob.dm

@@ -174,8 +174,7 @@
 	return 0
 
 /mob/proc/Life()
-	set waitfor = 0
-	return
+	set waitfor = FALSE
 
 /mob/proc/get_item_by_slot(slot_id)
 	return null