glTF exporter: Big gltf primitive extraction refactoring + Blender attributes export

io_scene_gltf2/__init__.py

io_scene_gltf2/blender/com/gltf2_blender_conversion.py

io_scene_gltf2/blender/exp/gltf2_blender_extract.py

.../blender/exp/gltf2_blender_gather_primitive_attributes.py

...cene_gltf2/blender/exp/gltf2_blender_gather_primitives.py

...f2/blender/exp/gltf2_blender_gather_primitives_extract.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_tree.py