Working at unraveling and breaking dependencies inside of src/Model.

Working at unraveling and breaking dependencies inside of src/Model.
I split the FGModelPlacement code out into it's own set of source files.
I created two versions of the fgLoad3DModel() routine.  One that is
  unecumbered by a panelnode dependency and one that is.  acmodel.cxx is
  the only place that needs to load an aircraft with instrument panels.
model.[ch]xx are now pretty much free to move over into simgear.
loader.[ch]xx should be able to follow closely behind.
This will be a big step towards being able to move the material management
code over into simgear.