Split runway and taxiway into separate classes, with a (new) common base.

Split runway and taxiway into separate classes, with a (new) common base.
This makes taxiways smaller (important since at present there are so many).
Restructure the apt.dat parsing code to use a helper class instead of one long
function, and to do less work when parsing the file.

Some of these ideas come from Yon Uriarte's patches - thanks Yon.