Add per-bone meta to Skeleton3D

[demolke]

• Bones are not represented as Nodes and don't inherit get/set meta functionality from Object, so the skeleton has to carry the meta information similarly to how other per-bone properties are handled.
• Adds support for GLTF import/export

This was split off of #86183 which only deals with import/export of metadata for nodes, meshes and materials - all of which do inherit from Object and therefore support metadata already.

Supporting the same for bones requires changes to Skeleton3D datastructure, so it's better to have the discussion separate.

[demolke]

I would like to get #86183 merged first, so that this can be rebased and will be only one commit. Did not figure out how to tell github that this PRs diffbase is #86183

[AThousandShips]

There's no such system AFAIK, just make it clear in the PR description that it depends on that PR and the production team and reviewers will keep that in mind

[lyuma]

I like this idea because it enables round-tripping of GLTF extras.

Because we have heavily modified the skeleton class late in the release cycle already, I'm a bit hesitant to add another big change to the skeleton (adding another property per-bone). I think @TokageItLab would need to review the skeleton changes to make sure both the serialization and the API changes ok, but I don't want to put more on the plate for 4.3 this late in the cycle.

I'd feel more comfortable trying the original PR #86183 in for this release cycle, understanding that we won't have full round tripping of skeleton bone extras for now... and depending on feedback and when we have time, we can also add the skeleton bone support in the future.

That said, the code looks reasonably simple if I read only the second commit (since it's stacked on the other PR)... if Tokage approves it and we think the timing works, I think it's ok.

[demolke]

I've uploaded a new version which fixes deleting of meta and also contains the UI portion of the change. I've separated the "Add Metadata" dialog logic from Node into a separate Dialog and linked it to both node meta and bone meta.

[AThousandShips]

LGTM

[akien-mga]

Thanks!