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/modules/mob/abstract/observer/observer.dm

@@ -3,7 +3,8 @@
 	desc = "It's a g-g-g-g-ghooooost!" //jinkies!
 	icon = 'icons/mob/mob.dmi'
 	icon_state = "ghost"
-	layer = OBSERVER_PLANE
+	layer = OBSERVER_LAYER
+	plane = OBSERVER_PLANE
 	stat = DEAD
 	density = 0
 	canmove = 0