As an opt-in API, allow clients to request partial results, with a time-bounded cutoff. Use this to keep the MapWidget responsive even when many airports are being added to the cache (e.g., zooming out or panning rapidly when zoomed out)
Cache the parsed navigation and airport data in a binary file to reduce
startup times and memory consumption (since only referenced FGPositioned
elements are held in memory).
Data will be reimported when the mod-time of any input file is changed.
If a global file is changed (nav.dat, awy.dat, apt.dat, etc), the cache
will be completely rebuilt, which takes approximately 30 seconds on
moderate hardware. (Future work may reduce this).
