Revert class name back to "valueview"

Constructing widgets works different in the currently used version of jQuery. Looks like the option is ignored now since `this.widgetBaseClass` started to return `valueview-valueview`. Which broke most gadgets on wikidata.org.

I'm not 100% sure if this is a proper fix. "Somebody" needs to run the browser tests before this is merged. ;-)

Note: The same problem may be present in the RankSelector widget. We should check this.

Bug: 66883