- Runways are now part of an airport, instead of a separate list
- Runways are no longer represented as a boring struct, but as a class
of their own.
-Improved runway access to unify various runway access methods.
Various other patches that have been lingering around for a while:
* Moved trafficcontrol.[ch]xx from the Airports directory to ATC, where
it really belongs.
* AI aircraft will request startup clearance, and ground control will
approve.
* Starting AI Aircraft will be pushed back to a predefined holding point
on the ground network, and wait a while before taxiing out to the runway
