Branch for development of newgrf airports, seaports, landports.

Initial aim of this branch is to replace the hardcoded airports (finite state machines (FSMs) and graphics layouts), with a newgrf-loadable system similar to newstations. As the airport contains an FSM, it is more active than the newstations, and requires numerous extensions to the newstation specification.

Although this will at first only apply to airports and aircraft, the code will be designed to apply to all non-train vehicles; thus it should eventually allow seaports, bus terminii, and lorryparks to be coded both graphically and actively.