Mobs can be incorporeal (#6791)

* Mobs can be incorporeal

Prevents step noises, getting shot, etc. Basically living ghosts. Feel free to add more places this should check, but I think this is a majority.

* Fix missing incorporeal floatyness

code/game/objects/effects/decals/Cleanable/humans.dm

@@ -70,6 +70,8 @@ var/global/list/image/splatter_cache=list()
 		desc = initial(desc)
 
 /obj/effect/decal/cleanable/blood/Crossed(mob/living/carbon/human/perp)
+	if(perp.is_incorporeal())
+		return
 	if (!istype(perp))
 		return
 	if(amount < 1)