surface: Add navigation lights to the airport (and SurfaceBuilding).

Also fixed a bug that SurfaceBuilding's landOffset property is summed
to the model graphics twice.
surface: Make F15 use shared taxiing AI as the A10's.
surface: Aerial auto pilot AI can take off from ground.

demo7.nut demonstrates the AI by extending taxiing path.
surface: Add dedicated Airport class for the airport structure.

It has been SurfaceBuilding, but it's gonna have functions unique to
an airport.
surface: Make auto piloted airplanes retract landing gears after take off.
surface: Fix bug that Aerial derived classes incorrectly share hitboxes.

The hitboxes should be independent to class type.  Keeping the shape as a
static variable in a member function of Aerial have prevented to achieve
the independency.
surface: Refine A10's model to have control surfaces visible.

Also added landing gear graphics and motions.  Hardpoint positions are
adjusted, too.