Split the Document Manager extension into multiple plugins

[jtpio]

References

Follow-up to #12349

Fixes #12965

This will help with jupyter/notebook#6446 to avoid duplicating too much logic in the Notebook v7 code base.

Code changes

Split some of the docmanager extension logic into multiple plugins so they are easier to reuse and compose downstream.

New plugins:

• @jupyterlab/docmanager-extension:opener: a plugin to open document widgets
• @jupyterlab/docmanager-extension:contexts: handle dirty contexts

TODO

• Split plugins
• Add docstrings
• Fix tests
• Fix examples
• Add to extension migration guide

User-facing changes

This fixes an issue introduced in #12349 not being able to create a new view for a document widget:

This was because the name opener was used in the code, but was not referring to the widget opener but implicitly to window.opener instead: https://developer.mozilla.org/en-US/docs/Web/API/Window/opener

Backwards-incompatible changes

None at the API level

• The IDocumentWidgetOpener interface also now defines an opened signal that is emitted when a widget is opened.
• Some plugins have their dependencies changed.

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[jtpio]

cc @fcollonval for review, since you also had a look at this in #12966

[fcollonval]

Thanks @jtpio

I have a minor suggestion. Otherwise this is good to go.

[fcollonval]

@jtpio sorry I missed the instantiation of this._signal. So instead of Object.freeze an anonymous class would be better.

[jtpio]

ah right, at least CI caught it.

I'll push a fix.

[fcollonval]

Thanks