vehicle flatbed trailers

[mlangsdorf]

add a new vehicle part: flatbed carrier.
each flatbed carrier has its own carried_parts vector.
activate a flatbed carrier, aim at an adjacent vehicle, and if there are sufficient adjacent flatbed trailers to store that vehicle, move each of that vehicle's tile's worth of parts into the flatbed's carried_parts.
Rebuild the vehicle on unload, starting from the upper left flatbed carrier.
Damage to a flatbed carrier carries on to the carried_parts.