Footstep sounds

This adds a system for footstep sounds
 - Only xenomorphs and humans currently use it, could expand to robutts
   and simple_animals easily, but haven't.
 - Only enabled for three turf types currently:
  - Plating (ala Maintnance)
  - Carpets
  - Wooden floors

code/modules/mob/living/carbon/alien/alien.dm

@@ -295,3 +295,16 @@ Des: Removes all infected images from the alien.
 #undef HEAT_DAMAGE_LEVEL_1
 #undef HEAT_DAMAGE_LEVEL_2
 #undef HEAT_DAMAGE_LEVEL_3
+
+/mob/living/carbon/alien/handle_footstep(turf/T)
+	if(..())
+		if(T.footstep_sounds["xeno"])
+			var/S = pick(T.footstep_sounds["xeno"])
+			if(S)
+				if(m_intent == "run")
+					if(!(step_count % 2)) //every other turf makes a sound
+						return 0
+				if(leaping)
+					return 0
+				playsound(T, S, 6, 1, -(world.view/2)) //xenos are quiet fuckers
+	return 0

code/modules/mob/living/carbon/human/human_movement.dm

@@ -81,4 +81,22 @@
 			if(!has_gravity(loc))
 				return
 			var/obj/item/clothing/shoes/S = shoes
-			S.step_action(src)
+			S.step_action(src)
+
+/mob/living/carbon/human/handle_footstep(turf/T)
+	if(..())
+		if(T.footstep_sounds["human"])
+			var/S = pick(T.footstep_sounds["human"])
+			if(S)
+				if(m_intent == "run")
+					if(!(step_count % 2)) //every other turf makes a sound
+						return 0
+				playsound(T, S, shoes ? 15 : 7, 1, shoes ? 0 : -(world.view/2))
+					//this looks insane but it actually is quite simple
+					//play sound at turf T, using sound S
+					//volume = 15 w/ shoes, volume = 7 w/o shoes
+					//do use vary
+					//add negative world.view/2 if not wearing shoes
+				return 1
+
+	return 0

code/modules/mob/living/living.dm

@@ -492,10 +492,18 @@
 	if (s_active && !( s_active in contents ) && get_turf(s_active) != get_turf(src))	//check !( s_active in contents ) first so we hopefully don't have to call get_turf() so much.
 		s_active.close(src)
 
+	handle_footstep(loc)
+	step_count++
+
 	if(update_slimes)
 		for(var/mob/living/carbon/slime/M in view(1,src))
 			M.UpdateFeed(src)
 
+/mob/living/proc/handle_footstep(turf/T)
+	if(istype(T))
+		return 1
+	return 0
+
 /*//////////////////////
 	START RESIST PROCS
 *///////////////////////

code/modules/mob/living/living_defines.dm

@@ -51,4 +51,5 @@
 	var/list/icon/pipes_shown = list()
 	var/last_played_vent
 
-	var/list/datum/action/actions = list()
+	var/list/datum/action/actions = list()
+	var/step_count = 0