- add a button to the main dialog, explaining how to adjust the
fg-root path via the GUI
- tweak the GUI flow to support explicitly changing the path even
when the default path is acceptable.
- change launcher to examine the scenery paths and hence load
ground-net files for airports to populate parking data.
- refactor ground-net XML parsing to use FGGroundNetwork only, not
AirportDynamics.
- change parenting of GroundNetwork to Airport, since it contains
immutable data now.