Facility vehicles 001 - Road roller

Adds the first road construction vehicle on my list - road roller. Based on a flatbed truck (tried to emulate the single-man cabin to some extent), it features a row of heavy duty drums at the front and at the back. These can be deconstructed only, yielding some steel plating and debris.

Wanted to make it spawn in the public works (can it be done ?), but it seems that the building is non-mapgen.
These should probably have the `WHEEL` flag, since they're intended to flatten all in their path.  ;-)