Remove all name and spatial queries from FGNavList. All remaining queries are

Remove all name and spatial queries from FGNavList. All remaining queries are
by frequency (which makes sense), and use the FGPositioned spatial data if
required. As a result, the marker beacon list is gone (since beacons are only
searched spatially). In the process, clean up various minor things - most
notably, all the 'airport-related' navaids (ILS, GS, LOC, and the beacons) now
store a FGRunway* instead of an airport id string. This is more precise, and
saves string allocations.