FBX import doesn't preserve joint names

When importing an FBX with joints, the FBX import applies its own naming scheme "n0, n1, n2, ..." to joint names, rather than using the joint names stored in the FBX file.

This makes it difficult to compose FBX skeletons with animations from different file formats...

For example, using `model_skeleton.fbx` from https://github.com/ubisoft/ubisoft-laforge-animation-dataset/blob/master/fbx/model_skeleton.fbx, we get:

... rather than Hips, Spine1, etc...