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.
