Emit changed signal from output area when an output is updated

[bollwyvl]

References

• fixes vega (any?) MIME renderers do not update saved output MIME bundle #14781

Code changes

• emit changed from OutputAreaModel when an output is updated with setData (or changes in any other way, presumably)
• add browser-based acceptance test that verifies changes are updated by e.g. vega

User-facing changes

• users should see fallback/metadata from mime renderers which use setData

Backwards-incompatible changes

• ???

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[bollwyvl]

well that was clearly wrong...

[fcollonval]

Some subtle elements, as the fixture tmpPath is set at the file level, all tests will be executed within the same working folder. So as the file was saved, the initial content changed between the 1st and 2nd test.

I pushed a fix to also use the files API instead of fs as getting the local path is a tough business (the root directory used is a dynamically generated temporary directory).

Hopefully the CI will be green(er 😉)

[bollwyvl]

Thanks for the alley-oop 🏀, there's clearly more about galata i don't know than otherwise. Likely this pattern of "check what ends up on disk after doing stuff" could be applied in other places.

[fcollonval]

Thanks @bollwyvl

[fcollonval]

CI failure are not related

[fcollonval]

@meeseeksdev please backport to 4.0.x