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
Oh good god what are you doing.  If you really want to put every goddamn path in the code in here, feel free.  Otherwise, put an "else" after the welding tool stuff and call the host turfs attackby proc with the same arguments.