glTF importer: prettify bones: manage more readable bone rotation

io_scene_gltf2/__init__.py

io_scene_gltf2/blender/com/gltf2_blender_math.py

io_scene_gltf2/blender/imp/gltf2_blender_animation_bone.py

io_scene_gltf2/blender/imp/gltf2_blender_animation_node.py

io_scene_gltf2/blender/imp/gltf2_blender_node.py

io_scene_gltf2/blender/imp/gltf2_blender_vnode.py