model.[ch]xx:

model.[ch]xx:
  add abstract class SGModelData. If a pointer to such a class is handed over
  to sgLoad3DModel, then its modelLoaded() method is called with path, property
  node and branch. And then it's added to the scene graph so that it's
  destroyed when the model branch is removed from the graph.

modellib.[ch]xx:
  only cache objects when asked to. This is the case for OBJECT_SHARED
  and random objects (like before), but no longer for OBJECT_STATIC.
  These are now removed from the graph when they are "out of sight". This
  prevents accumulation of static models, and makes destroying model data
  possible (e.g. removing Nasal modules)

matmodel.cxx:
  set cache flag for random objects (same behavior as before)