- Moved some property specific code into simgear/props/

- Moved some property specific code into simgear/props/
- Split out the condition code from fgfs/src/Main/fg_props and put it
  in it's own source file in simgear/props/
- Created a scene subdirectory for scenery, model, and material property
  related code.
- Moved location.[ch]xx into simgear/scene/model/
- The location and condition code had dependencies on flightgear's global
  state (all the globals-> stuff, the flightgear property tree, etc.)  SimGear
  code can't depend on it so that data has to be passed as parameters to the
  functions/methods/constructors.