Modify existing RoadDefs (through DefModExtension), to add all information necessary to make roads buildable

• This will allow the definition of costs in the RoadDef (or at least, linked to the RoadDef), so generated roads have a cost as well, which can help with cost rebate when upgraded

• A "Built" property will allow for a different way of drawing roads in maps, so that other objects are not drawn on top of newly-built roads. They can look "newer" as well

• Any change of construction cost can be centralised in one XML

• Adding new roads (like GlitterRoads) is easier

• Cost setting & calculating resource consumption can be done within this DefModExtension