Small refactor of curtains

This commit ports Baystation12/Baystation12#8921.
Curtains now use SHOWER_OPEN_LAYER and SHOWER_CLOSED_LAYER instead of
manual layer definitions. Two new default shower curtains have been
added, Engineering and Security. They are not currently avalible on the
map due to the fact that Paradise's map, as of this commit, does not
have any showers anywhere but the dorms & engineering satellite.

code/game/objects/structures/curtains.dm

@@ -1,14 +1,17 @@
+#define SHOWER_OPEN_LAYER OBJ_LAYER + 0.4
+#define SHOWER_CLOSED_LAYER MOB_LAYER + 0.1
+
 /obj/structure/curtain
 	icon = 'icons/obj/curtain.dmi'
 	name = "curtain"
 	icon_state = "closed"
-	layer = MOB_LAYER + 0.1
+	layer = SHOWER_CLOSED_LAYER
 	opacity = 1
 	density = 0
 
 /obj/structure/curtain/open
 	icon_state = "open"
-	layer = OBJ_LAYER
+	layer = SHOWER_OPEN_LAYER
 	opacity = 0
 
 /obj/structure/curtain/bullet_act(obj/item/projectile/P, def_zone)
@@ -27,10 +30,10 @@
 	opacity = !opacity
 	if(opacity)
 		icon_state = "closed"
-		layer = MOB_LAYER + 0.1
+		layer = SHOWER_CLOSED_LAYER
 	else
 		icon_state = "open"
-		layer = OBJ_LAYER
+		layer = SHOWER_OPEN_LAYER
 
 /obj/structure/curtain/black
 	name = "black curtain"
@@ -45,3 +48,12 @@
 	name = "shower curtain"
 	color = "#ACD1E9"
 	alpha = 200
+
+/obj/structure/curtain/open/shower/engineering
+	color = "#FFA500"
+
+/obj/structure/curtain/open/shower/security
+	color = "#AA0000"
+
+#undef SHOWER_OPEN_LAYER
+#undef SHOWER_CLOSED_LAYER