KMLExport: Added airspeed to export map

Previously the airspeed field was set to -1.

In addition, the arrow color now represents calibrated airspeed, whereas the line color represents groundspeed. This is perhaps inappropriate for ground vehicles, so should be vehicle-type selectable in the future.

Also changed timespans to work on the flight path, making it easier to follow the vehicle's trajectory when using Google Earth's playback function.