Fix windowed notebook and ToC getting broken on reloading from disk

[krassowski]

References

Fixes #15415

Code changes

• adds a test case for headingsChanged being emitted by NotebookToCModel on cell insertion, deletion and reloading the model
• fixes the comparison of heading equality which was preventing updates after reloading file from disk by introducing a new TableOfContentsModel.isHeadingEqual() method which can be overridden in subclasses
• fixes windowing breaking on revert by adding a guard widget !== this._willBeRemoved in the conditional check sensing if the active cell should be softly detached (or removed)

User-facing changes

ToC/windowing works well when pressing "Revert" button.

Backwards-incompatible changes

None

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[jtpio]

Thanks!

I tried with the built assets from the check_release run and it looks like it fixes the issue.