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

code/modules/mob/mob_defines.dm

@@ -1,11 +1,11 @@
 /mob
+	datum_flags = DF_USE_TAG
 	density = TRUE
 	layer = MOB_LAYER
 	animate_movement = 2
 	flags_1 = HEAR_1
 	hud_possible = list(ANTAG_HUD)
 	pressure_resistance = 8
-	use_tag = TRUE
 	var/lighting_alpha = LIGHTING_PLANE_ALPHA_VISIBLE
 	var/datum/mind/mind
 	var/list/datum/action/actions = list()