Convert FGRunway to be heap-based, and inherit FGPositioned. This is a large, ugly change, since FGRunway was essentially a plain struct, with no accessors or abstraction. This change adds various helpers and accessors to FGRunway, but doesn't change many places to use them - that will be a follow up series of patches. It's still a large patch, but outside of FGAirport and FGRunway, mostly mechanical search-and-replace.
An interesting part of this change is that reciprocal runways now exist as independent objects, rather than being created on the fly by the search methods. This simplifies some pieces of code that search for and iterate runways. For users who only want one 'end' of a runway, the new 'isReciprocal' predicate allows them to ignore the 'other' end. Current the only user of this is the 'ground-radar' ATC feature. If we had data on which runways are truly 'single-ended', it would now be trivial to use this in the airport loader to *not* create the reciprocal.
