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/tgui/tgui.dm

@@ -35,6 +35,7 @@
 	var/list/datum/tgui/children = list() // Children of this UI.
 	var/titlebar = TRUE
 	var/custom_browser_id = FALSE
+	var/ui_screen = "home"
 
  /**
   * public
@@ -216,7 +217,7 @@
 	var/list/config_data = list(
 			"title"     = title,
 			"status"    = status,
-			"screen"	= src_object.ui_screen,
+			"screen"	= ui_screen,
 			"style"     = style,
 			"interface" = interface,
 			"fancy"     = user.client.prefs.tgui_fancy,
@@ -277,7 +278,7 @@
 			initialized = TRUE
 		if("tgui:view")
 			if(params["screen"])
-				src_object.ui_screen = params["screen"]
+				ui_screen = params["screen"]
 			SStgui.update_uis(src_object)
 		if("tgui:link")
 			user << link(params["url"])