Implement the Nearest Airports PFD softkey, displaying a scrollable
list of the closest 25 airports within 200nm.
Also fix a couple of minor bugs and add self-checking to the
GroupElement
- Pre-load nearest airports to remove significant freeze when the
NearestAirports page is accessed for the first time.
- Update multikey support to provide easy access to commonly used MFD
pages and access to softkeys.
