Facility vehicles 001 - Road roller [WIP]

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.
Yikes!  two weight lines.  (One of the reasons I posted those links: the drum on the smaller roller weighed a little over 800000 grams per segment.  Top speed was 7.5 MPH.  The road roller should be a **heavy** monster.)