feat(ui): layers metadata recall

[psychedelicious]

Summary

Support metadata recall for layers.

TODO:

• [ ] Do not store ephemeral layer state in metadata (e.g. 'uploadedMaskImage', maybe bboxNeedsUpdate)
• [ ] Check images for existence? May as well implement this for all metadata recallers that use an image while I'm at it...
• [ ] Decide how to handle metadata from layers into canvas
• [ ] Decide whether to store both the old-style metadata and layers metadata - I don't think I want to store both...
• [ ] Add versioning to metadata
• [ ] Test thoroughly

Related Issues / Discussions

n/a

QA Instructions

When ready for review, use control layers and check the metadata tab. Try recalling an individual layer, or use all to get all layers.

Merge Plan

n/a

Checklist

• [x] The PR has a short but descriptive title, suitable for a changelog
• [x] Tests added / updated (if applicable)
• [ ] Documentation added / updated (if applicable)