nbagg: Use OutputArea event to trigger figure close.

[QuLogic]

PR Summary

When converting from jQuery to vanilla JavaScript, the trigger for the figure closure was set to a non-existent event name, as the 'remove' event is jQuery-specific.

As there is no DOM node removal event (unless using MutationObserver), connect this handler to the notebook's Cell OutputArea's 'cleared' event. This is triggered both by re-running a cell, and by using the Clear Output menu option.

Fixes #18447.

PR Checklist

• [n/a] Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• [n/a] New features are documented, with examples if plot related.
• [n/a] Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and pydocstyle<4 and run flake8 --docstring-convention=all).
• [n/a] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [n/a] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[dopplershift]

Unfortunately, the test failures look real... Ah, I see it's just the current nbconvert problem.

[QuLogic]

I pushed a (hopefully safe) pin for nbconvert, so we can fix CI as well.

[tacaswell]

Checked this manually and it work is both brave (chrome-derivative) and firefox.

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout v3.3.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 f24a1e250de15e79b0212960aba2c61386bbeb16

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #18454: nbagg: Use OutputArea event to trigger figure close.'

4. Push to a named branch :

git push YOURFORK v3.3.x:auto-backport-of-pr-18454-on-v3.3.x

5. Create a PR against branch v3.3.x, I would have named this PR:

"Backport PR #18454 on branch v3.3.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.