Visual / layering fixes (#19547)

Package of a few visual changes to make things prettier-er. Also added a
helper for mapping purposes.

  - bugfix: "Carpets now show proper edges again."
- bugfix: "Fixed layering of several map items, like disposal pipes,
that were previously buried underneath turfs in the map editor (and were
invisible as a result)."
- bugfix: "Fixed observer sprites occasionally going underneath certain
objects. Their layering has been fixed."
- bugfix: "Reinforced plastic tables now show up correctly in the map
editor."
- rscadd: "Added a special 'lock' door helper that automatically locks
doors underneath it so mappers don't have to edit variables/icon states
manually."


![image](https://github.com/Aurorastation/Aurora.3/assets/55491249/53d97049-0104-491a-9a56-4d677afec536)

![image](https://github.com/Aurorastation/Aurora.3/assets/55491249/a26c9cf6-1ea9-4654-bc5c-954c878efda3)

![image](https://github.com/Aurorastation/Aurora.3/assets/55491249/6dbee17e-6a24-4cc9-b194-4c46530020be)

Fixes #18762 
Fixes #18840 
Fixes #19101

code/game/objects/structures/full_window_frame.dm

@@ -5,7 +5,7 @@
 	icon_state = "window_frame"
 	color = COLOR_GRAY20
 	build_amt = 4
-	layer = ABOVE_TABLE_LAYER
+	layer = WINDOW_FRAME_LAYER
 	anchored = TRUE
 	density = TRUE
 	climbable = TRUE

code/game/objects/structures/grille.dm

@@ -10,14 +10,14 @@
 	anchored = TRUE
 	obj_flags = OBJ_FLAG_CONDUCTABLE | OBJ_FLAG_MOVES_UNSUPPORTED
 	explosion_resistance = 1
-	layer = BELOW_OBJ_LAYER
+	layer = BELOW_WINDOW_LAYER
 	var/health = 10
 	var/destroyed = 0
 
 /obj/structure/grille/over
 	name = "over-frame grille"
 	icon = 'icons/obj/smooth/window/grille_over.dmi'
-	layer = BELOW_OBJ_LAYER
+	layer = BELOW_WINDOW_LAYER
 	smoothing_flags = SMOOTH_MORE
 	canSmoothWith = list(
 		/turf/simulated/wall,

code/game/objects/structures/lattice.dm

@@ -8,7 +8,7 @@
 	density = FALSE
 	anchored = TRUE
 	w_class = ITEMSIZE_NORMAL
-	layer = LATTICE_LAYER
+	layer = ABOVE_TILE_LAYER
 	obj_flags = OBJ_FLAG_MOVES_UNSUPPORTED
 	smoothing_flags = SMOOTH_MORE
 	canSmoothWith = list(

code/game/objects/structures/plasticflaps.dm

@@ -5,7 +5,7 @@
 	icon_state = "plasticflaps_preview"
 	density = 0
 	anchored = 1
-	layer = ABOVE_HUMAN_LAYER
+	layer = UNDERDOOR
 	explosion_resistance = 5
 	build_amt = 4
 	color = COLOR_GRAY20 // ideally this would get_step() the material color from nearby walls but this works for now.