Apparently we were already getting this info because it's a unit like
any other according to the event system, so if runways were actually
sufficiently damaged we'd emit a ton of exceptions.
This doesn't do anything yet, but tracks the damage state. Will add the
ability to repair them next, and then finally make the damage render the
runway inoperable.
Generated units are added to this during mission generation so we can
map destroyed units back to the data that generated them. Currently only
implemented for aircraft as a proof of concept.
