GLTF: Add append_gltf_node to GLTFState

[aaronfranke]

This PR adds append_gltf_node to GLTFState, and append_child_index to GLTFNode.

The append_gltf_node method is mostly the same as the previous internal _create_gltf_node, but cleaned up. Here are some of the differences:

• append_gltf_node is responsible for determining the index it's inserting into, and returns it. The API of _create_gltf_node basically just trusted that the caller did not get it wrong, now it's always correct by design (to be clear the old code behaved correctly, but only because it was used correctly in the one place it was used).
• append_gltf_node will handle setting the given node as a root node, if the parent was -1. This responsibility was previously handled by GLTFDocument::_convert_scene_node.
• append_gltf_node has detailed documentation explaining when it can be called and what input is valid.
• If append_gltf_node is used to append the current node during convert_scene_node extension code, then it won't also be appended by GLTFDocument. This allows extensions to "move" a node to another parent on export.
• If the returned node's index is -1, it will be appended automatically. If -2 or less, it will be excluded.

I am proposing to expose this method because I need it in a GLTFDocumentExtension class for export logic. Currently the glTF exporter code expects that one Godot node should become one glTF node, and there's no (easy) way to make multiple glTF nodes for one Godot node. With this method exposed, now it's easy. I need this to allow my GLTFDocumentExtension class to export multiple glTF nodes from one Godot node, and I have tested this PR with that project.

As for adding append_child_index to GLTFNode, this doesn't work: gltf_node.children.append(5). You would need to make a local variable, then append to it, then set it back. That's 3 lines for what could just be 1, so that's why I'm adding this method to allow doing gltf_node.append_child_index(5).

[fire]

Looks good to me.

In a separate pr, I would appreciate to see how much can be done to FBXDocument, but I understand it's not directly here. We are trying to restore FBXDocumentExtensions.

[akien-mga]

Thanks!