Fix multiple bugs with catwalks.
* Fixes catwalks not having space underneath them
* Fixes not being able to build catwalks
* Catwalks destructible with screwdriver
* Catwalks autoupdate icons with neighbors

Conflicts:
	code/game/turfs/simulated/floor.dm
	code/game/turfs/simulated/floor_types.dm
	html/changelog.html