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/keybindings/focus.dm

@@ -1,20 +1,8 @@
-/datum
-	var/list/focusers //Only initialized when needed. Contains a list of mobs focusing on this.
-
 /mob
 	var/datum/focus //What receives our keyboard inputs. src by default
 
 /mob/proc/set_focus(datum/new_focus)
 	if(focus == new_focus)
 		return
-
-	if(new_focus)
-		if(!new_focus.focusers) //Set up the new focus
-			new_focus.focusers = list()
-		new_focus.focusers += src
-
-	if(focus)
-		focus.focusers -= src //Tell the old focus we're done with it
-
 	focus = new_focus
-	reset_perspective(focus) //Maybe this should be done manually? You figure it out, reader
+	reset_perspective(focus) //Maybe this should be done manually? You figure it out, reader