Splits wall layer into three (#85901)

## About The Pull Request

Turns 
`ON_WALL_LAYER`
into
`FLAT_ON_WALL_LAYER`
`ON_WALL_LAYER`
`HIGH_ON_WALL_LAYER`

Where `FLAT_ON_WALL_LAYER` is meant for lower-priority wall mounts like
signs and posters
`ON_WALL_LAYER` is default
and `HIGH_ON_WALL_LAYER` is for stuff that "hang over" the wall

Also makes the incident display actually wall mounted

## Why It's Good For The Game

I noticed this while doing mapping and I thought it was a really cool
effect


![image](https://github.com/user-attachments/assets/93f33d54-2cda-41d1-abe6-d2035e0284c7)

Unfortunately this effect was a coinflip because all wall mounts were on
the same layer. Sometimes it'd look like this


![image](https://github.com/user-attachments/assets/59c3c6a7-561f-4ef6-813b-3cd06b295372)

So this allows us to do this kinda stuff consistently. 
Also has the added effect of letting us "de-prioritize" stuff like
posters, so we can hang stuff *over* posters and signs, which could be
useful.

## Changelog

🆑 Melbert
qol: Some wall mounts will now consistently layer over others (light
switches and cameras, notably, should always layer above other mounts
like signs and status displays)
/🆑

code/modules/mapfluff/ruins/lavalandruin_code/elephantgraveyard.dm

@@ -90,11 +90,11 @@
 	dispensedreagent = /datum/reagent/fuel/oil
 
 // This is a hole
-/obj/structure/sink/oil_well/find_and_hang_on_wall()
+/obj/structure/sink/oil_well/find_and_hang_on_wall(directional, custom_drop_callback, wall_layer)
 	return
 
 /obj/structure/sink/oil_well/Initialize(mapload)
-	.=..()
+	. = ..()
 	create_reagents(20)
 	reagents.add_reagent(dispensedreagent, 20)
 	//I'm pretty much aware that, because how oil wells and sinks work, attackby() won't work unless in combat mode.