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/game/gamemodes/nuclear/nuclearbomb.dm

@@ -79,14 +79,14 @@
 	anchored = TRUE //stops it being moved
 
 /obj/machinery/nuclearbomb/syndicate
-	use_tag = TRUE
+	datum_flags = DF_USE_TAG
 	//ui_style = "syndicate" // actually the nuke op bomb is a stole nt bomb
 
 /obj/machinery/nuclearbomb/syndicate/GenerateTag()
 	var/obj/machinery/nuclearbomb/existing = locate("syndienuke") in GLOB.nuke_list
 	if(existing)
 		stack_trace("Attempted to spawn a syndicate nuke while one already exists at [COORD(existing.loc)]")
-		use_tag = FALSE
+		datum_flags &= ~DF_USE_TAG
 		return
 	tag = "syndienuke"