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

(cherry picked from commit fa2a3a7fd5906c872a84e2e0d54aa4e4188392a7)

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