FlightGear - Flight Simulator FGAddon

Main:
- Payload system now works with the jetways, by enabling the checkboxes of the jetways you have connected. These checkboxes don't move the jetways, they only affect the boarding speed; passengers will not board if none of the stairs/jetways checkboxes are enabled.
- Added extra catering trucks.
- Add catering loading logic; catering loaders will load cabin trolleys to the catering galleys. Catering loading starts and stops with baggage loading.
- User now has the ability to save the weight properties and ground services state between sessions.
- Added 4th boarding staircase to increase boarding speed.
- Enable dynamic taxi light; because of the constraints of the ALS light spots, I have replaced one landing light with the taxi light and moved the other landing light to the center and made it brighter, so it provides a light spot for both landing lights.
- Replaced dynamic nav lights with dynamic left and right wing strobe lights and the belly beacon light (because these are brighter IRL and give a nice effect on the surrounding when flashing)

Also:
- Fix numerous erronous property references
- Fix model reference for fire trucks (made a mistake on those in the last commit)
- Add some previews