Consider using the `gltf` crate instead of rolling a custom GLTF handling

[djeedai]

See https://github.com/djeedai/weldr/pull/27/files#r1127095741

[djeedai]

Sadly the gltf crate is restricted to loading, and doesn't contain any editing not saving capability. There's a basic JSON serializing but no binary support as far as I can tell. Several users asked for editing, issues are still open.