fbx2nw: Generate at most one GR2 file
fbx2nw: Generate at most one GR2 file

fbx2nw will generate at most one GR2 file choosing between a GR2 with
skeletons or a GR2 with animation. The choosing is based on the
following:
- If the FBX has an animation, only generates a GR2 with animation.
- Otherwise, if the FBX has skeletons, only generates a GR2 with skeletons.

The generated GR2 file have no longer .skel.gr2 or .anim.gr2 extension
but just .gr2 extension.

Arbos/nwn2mdk,Arbos/nwn2mdk,Arbos/nwn2mdk