FBX export: Huge refactor of 'Model' part of the code, and fix exporting instances when org objects are not selected.

'Model' data (which can be Blender Object, DupliObject or Bone/PoseBone) are now
wrapped in an ObjectWrapper class that present the same API in all cases.
Hopefully this will make that part of the code easier to follow (and maintain/troublefix).

Also made var names a bit more consistent.

Note all this makes export slightly slower in worst cases (many actions to check against objects),
about 15% it seems. But tha 'export all actions' code need some more attention, maybe it's
possible to enhance its performances?