Rigging roundtrip

[julienduroure]

Fix #1662

• Changing default import mode to "BLENDER", avoiding changing bone axis. So you can reexport without any axis changed
• At import, it displays bones as sphere shaped, and relationship lines from root (no more from tail). The shape object is added in a special collection, not active in viewport
• This collection is excluded at export, to avoid exporting an object that was not in original imported glTF.

This should help roundtrip.

Additional reminder: Blender is not a glTF editor, and we could not be sure in 100% that data are not converted / changed, as Blender & glTF are not 100% compatible. We are doing our best to be able to import and then export without any major changed, but sometimes we can't.

[julienduroure]

TODO:

• Change option description?
• Tests
• Update Doc