Ensure that registerToolView returns the existing tool view if possible.

This partially reverts 9efeb310 but the
changed hunk in registerToolView there does not look correct anyways.

New tests where added that try to assert the proper behavior.

Note: Constness of ToolViewData is now ensured for the OutputWidget.
This further shows that we should refactor out its views member and
move it to the stnadard output view.

Thanks to David Narváez for showing this broken behavior.

REVIEW: 107126