KSEA Sea-Tac CFG Update

[Flightdoc]

Re-working all the walk in gates at Sea-Tac. A lot of transparent gates and walkway so its worth some extra effort it really shows off the immersion of GSX.  Cargo ramps also given attention. Sample video -- GSX Editor allows distinct route for board vs de-board I like it.. De-icing stations present near the tower as in earlier version. Cargo pilots use Ramp 4 for Fedex, Ramp 6 for UPS. Tested under SU14.

https://drive.google.com/file/d/1d9XNR15T9_G5MY3Y5vLbB_dzDY-DK98q/view?usp=sharing

https://drive.google.com/file/d/1rfOeq42k2JPEuRf-wJvb8Mr8aIvURvWj/view?usp=sharing