`DOMNode.workers` is a confusing alias

[rodrigogiraoserrao]

Currently, DOMNode.workers is a very confusing alias for App.workers.

When you type self.workers inside a widget, it really, really, REALLY looks like you are talking about the workers started by/associated with the given widget.
But you're actually talking about all app workers.

See #3615 (comment) for some extra context.

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.

[davep]

Big second.