Add support for de ice pads

[Awemeter]

With the winter season starting de-icing is becoming more and more common. It would be cool if gsx supported de icing at a pad instead of just at the gate.

[virtuali]

This has been discussed so many times, and yes, it's planned feature, but it's not as easy as it seems, since it surely requires adding editor customizations, to place the icing pad areas.