Flightgear flight plans

[albar965]

Support saving of flight gear flight plan format.
http://wiki.flightgear.org/Flightplan_XML_formats

This is one step towards #293 and then #138.

[albar965]

Fixed issue with wrong waypoint numbering in FlightGear export causing the simulator to crash. This happens only when using SID and/or STAR. Commit albar965/atools@c0b4427