Airspace : Update and make it look better

[Ysurac]

• In 2D mode it's really dirty
• Update it
• Add correct credits

[daviessm]

It doesn't seem to refresh properly when panning around the map or panning/tilting/zooming in 3D. I have to say it looks very cool in 3D though! Also I'd like to see it as a setting that can be enabled and remembered like airport pins. Perhaps move some of those settings to the layers tab, or rename them layers and experimental layers...something like that.

[Ysurac]

• Can't update airspaces for now : error in Belgium airspace in openair format.
• Settings are remembered