List every tree by its ID, never by its file

[kineticsystem]

Summary

• Every tree is its own row in the Trees tab, by its ID, even when its file holds several trees (e.g. pick_and_place.xml in the examples). The file shows on hover, in the breadcrumb and in the details.
• Only an empty or unreadable file is listed by its name, to be seen and fixed.
• A tree that shares its file has no delete button, which would delete the other trees too; the unsaved and problem markers show on every tree of the file.
• README and docs/ARCHITECTURE.md updated.

Test plan

• test.sh: type check and 131 tests pass.
• Checked in Chrome with the editor's own examples: Main, PickAndPlace, Recharge and Pick listed by their IDs, the delete button only on the trees alone in their files.

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