Empire Raid with vehicles

An Imperial attack. You'll usually encounter Bulldogs and Highwaymen, rarely Scythemen, and in very exceptional cases, Buryas and Mosquitos. Warning: the vehicles are not balanced; they are the same ones the player uses (I'm too lazy to rebalance them in the XML).

The empire's settlements will be able to defend themselves with vehicles.

Two new types of Imperial trade vehicles were added: a land convoy with guards, and a trade helicopter. The Cherokee will seek a clear area to land, prioritizing beacons if there is sufficient space.
Two more options were added to the communications console to summon these types of trade vehicles. The relationship cost for each was increased, and the arrival time was reduced.

A vehicle was created from the Warbird to test the airplane-like flight mode.