feat!: remove confusing app workers alias

[TomJGooding]

Closes #3619.

I suggest we rename DOMNode.workers to DOMNode.app_workers and/or make it so that DOMNode.workers retrieves all workers started by/associated with the given widget.

There doesn't seem much benefit of just renaming this property if only a shortcut for self.app.workers?

This PR simply removes this confusing alias, but I'm happy to look into workers only tied to the given widget with a bit of guidance.

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[darrenburns]

I agree with this change. The alias is definitely confusing. However it'll need @willmcgugan to approve.

(Also needs a breaking change noted in the changelog if approved)

[willmcgugan]

I think Widget.workers should refer to a pool of workers for the self widget, if that is useful. Otherwise, we can remove the alias altogether.

I do want to do another pass on workers in the near future.