Rework of the GLTF converter

[RespectableRuessel]

Hi @CBenghi,

Due to the need of a GTLF converter that supports separate primitive node hierarchies (issue #10),
I began to rewrite the current one here: dn9090/XbimGltf (with most work done in this commit).

The current status is that the new implementation is a lot faster (35%) because of multi-threading
and is restructured so that it allows for an easier implementation of new features like the separate node hierarchies.
I noticed some things in the original implementation that I cannot understand or that are unnecessary in my view but
stayed in the implementation to ensure compatibility.
Also I reimplemented the original Builder API to use the new XbimGltfBuilder under the hood.

I would be glad if you could take a look at the project.
In the next steps I will try to support the hierarchy feature.

Thanks!