Catwalk re-implementation

(Re)adds catwalks as structures which are treated similarly to floor tiles with regards to movement. They are placed by clicking on lattice with rods, and are removed using a wrench

Wires can be run through lattices and catwalks

External walkways are now composed of catwalks instead of hull plating

Fixes #6771

Preview:
http://i.imgur.com/p8y9ITC.png
http://i.imgur.com/8IEfXsK.png
http://i.imgur.com/9rR7rab.png
Ditto?  Maybe?  I assume cable coils handle that via attackby.