Another clean-up iteration: FGAirportList::search is gone, replaced by two

Another clean-up iteration: FGAirportList::search is gone, replaced by two
static FGAirport helpers. As a result, another global index goes away. Use
the helpers to avoid ugly FGPositioned down-casts in various places.

Also converts the environment/METAR code to deal with FGAirport pointers,
instead of string identifiers, and contains work-in-progress code to implement
the AirportList dialog using FGPositioned. This isn't enabled yet for various
reasons, but is the final piece to allow FGAirportList to be removed.