Unifies datum var definitions (#2237)

code/datums/components/component.dm

@@ -66,8 +66,6 @@
 /datum/component/proc/OnTransfer(datum/new_parent)
 	return
 
-/datum/var/list/datum_components //list of /datum/component
-
 /datum/proc/SendSignal(sigtype, ...)
 	var/list/comps = datum_components
 	. = FALSE

code/datums/datum.dm

@@ -0,0 +1,31 @@
+/datum
+    var/gc_destroyed //Time when this object was destroyed.
+    var/list/active_timers  //for SStimer
+    var/list/datum_components //for /datum/components
+    var/ui_screen = "home"  //for tgui
+
+#ifdef TESTING
+    var/running_find_references
+    var/last_find_references = 0
+#endif
+
+// Default implementation of clean-up code.
+// This should be overridden to remove all references pointing to the object being destroyed.
+// Return the appropriate QDEL_HINT; in most cases this is QDEL_HINT_QUEUE.
+/datum/proc/Destroy(force=FALSE)
+	tag = null
+	var/list/timers = active_timers
+	active_timers = null
+	for(var/thing in timers)
+		var/datum/timedevent/timer = thing
+		if (timer.spent)
+			continue
+		qdel(timer)
+	var/list/dc = datum_components
+	for(var/I in dc)
+		var/datum/component/C = I
+		C._RemoveNoSignal()
+		qdel(C)
+	if(dc)
+		dc.Cut()
+	return QDEL_HINT_QUEUE