Move some stuff from different layers to different planes

Moves everything on a 15+ layer to a plane. So now you get screen catcher (-99, was already on a plane), lighting (15), effects that ignore lighting (16), fullscreen UI effects (18), screen objects used to build the UI (19), actual equipment in the UI slots (20), and everything else (0).

Also created a file to contain plane and layer defines for hopeful eventual use.

Hopefully this doesn't change anything now but does enable some nifty new features in the future.

code/modules/lighting/lighting_system.dm

@@ -27,7 +27,6 @@
 */
 
 #define LIGHTING_CIRCULAR 1									//Comment this out to use old square lighting effects.
-#define LIGHTING_LAYER 15									//Drawing layer for lighting
 #define LIGHTING_CAP 10										//The lumcount level at which alpha is 0 and we're fully lit.
 #define LIGHTING_CAP_FRAC (255/LIGHTING_CAP)				//A precal'd variable we'll use in turf/redraw_lighting()
 #define LIGHTING_ICON 'icons/effects/alphacolors.dmi'
@@ -236,7 +235,7 @@
 /atom/movable/light
 	icon = LIGHTING_ICON
 	icon_state = LIGHTING_ICON_STATE
-	layer = LIGHTING_LAYER
+	plane = PLANE_LIGHTING
 	mouse_opacity = 0
 	blend_mode = BLEND_OVERLAY
 	invisibility = INVISIBILITY_LIGHTING
@@ -362,7 +361,6 @@
 		T.init_lighting()
 		T.update_lumcount(0)
 
-#undef LIGHTING_LAYER
 #undef LIGHTING_CIRCULAR
 #undef LIGHTING_ICON
 #undef LIGHTING_ICON_STATE