Preserve skeletons which do not contain a mesh

[lyuma]

Generates an extra unused glTF skin for each node hierarchy with a FBXSkeleton, regardless of whether that node is connected to an existing skin.

Fixes #19

Should solve import for some Mixamo animations.
See also https://twitter.com/KenneyNL/status/1666528263890522126

[fire]

How should I test this is working?

[lyuma]

Download the characters from https://kenney.nl/assets/series:Animated%20Characters and confirm that they import correctly with a skeleton.

And then we need to make sure this doesn't cause a regression, since "sometimes the fbx skeleton is for advisory notice only" - similar to how the unused gltf joints are advisory only.

This change technically can't break anything in terms of glTF, since glTF is agnostic to bones vs nodes. But it could affect the generated Godot scene hierarchies which could lead to unexpected changes for some Godot users. We will only know through testing.

[fire]

I saw this work on discord.

We had concerned this will cause unforeseen consequences, but we are unable to test.

The blender gltf io addon was ok with gltf animations hacked with a skeleton skin.

We are unsure about the skeleton property @RevoluPowered, but they're advisory. So I think it's fine.