Add better airway support, fix various issues with VIA, and add
parsing / generation of ICAO route strings. Also fix the
serialisation of flight-plans with airway enroute segments, so these
can be restored correctly.
For now it is available in the positioned Nasal
module. FGAirport is fully ported, but especially
procedures/navaids and free functions are still
missing.
Convert the route-manager to use a flight-plan internally, and expose
flightplan, leg and procedure data to Nasal. Move the Level-D parser
into its own file.
