-tg- pull refactor

This commit does the following:
 - Overhauls how pull works. It is now based on a mob verb,
   mob/verb/pulled()
 - Makes the pull icon much more responsive. It is now updated whenever a
   mob starts or stops pulling an object, and instantly updates.
   - This required HUD changes. God help me.

code/modules/vehicle/train/trains/ambulance.dm

@@ -33,7 +33,6 @@
 /obj/vehicle/train/ambulance/engine/New()
 	..()
 	cell = new /obj/item/weapon/stock_parts/cell/high
-	verbs -= /atom/movable/verb/pull
 	key = new()
 
 /obj/vehicle/train/ambulance/engine/Move()

code/modules/vehicle/train/trains/cargo/cargo_train.dm

@@ -41,7 +41,6 @@
 /obj/vehicle/train/cargo/engine/New()
 	..()
 	cell = new /obj/item/weapon/stock_parts/cell/high
-	verbs -= /atom/movable/verb/pull
 	key = new()
 	var/image/I = new(icon = 'icons/obj/vehicles.dmi', icon_state = "cargo_engine_overlay", layer = src.layer + 0.2) //over mobs
 	overlays += I
@@ -279,10 +278,5 @@
 
 	if(!lead && !tow)
 		anchored = 0
-		if(verbs.Find(/atom/movable/verb/pull))
-			return
-		else
-			verbs += /atom/movable/verb/pull
 	else
-		anchored = 1
-		verbs -= /atom/movable/verb/pull
+		anchored = 1