FlightGear - Flight Simulator FGAddon

- Add sound to (de)boarding/(un)loading/catering/refueling procedures
- Allow (un)loading without connecting service vehicles (select payload and use the "Instant" button)
- Add de-icing trucks and full de-icing procedure (ground services dialog)
- Catering trucks overhaul: new independent (un)loading logic, updated payload dialog, models, animations (based on IDG A320)
- Add autodisconnect option (see Aircraft settings dialog): allow service vehicles to automatically disconnect when finished. (currently only implemented for catering vehicles, others will follow)
- Various bugfixes in ground service vehicles