Switch cables and pipes to use define constants instead of literals for layers.

Also fix a few things to be on the correct plane.  Basically the under-floor stuff is supposed to be on PLATING_PLANE.
Portable atmospherics go on object layer so they are above stationary machinery, given that they move around.
Fixed wires to be on top of pipes like they used to be, otherwise they'd be obscured too often.

code/modules/recycling/disposal.dm

@@ -663,6 +663,7 @@
 	var/dpdir = 0		// bitmask of pipe directions
 	dir = 0				// dir will contain dominant direction for junction pipes
 	var/health = 10 	// health points 0-10
+	plane = PLATING_PLANE
 	layer = DISPOSAL_LAYER	// slightly lower than wires and other pipes
 	var/base_icon_state	// initial icon state on map
 	var/sortType = ""