Add config schema for votingapi_widgets field

In #3501808: Remove config schema for the votingapi_widgets module, the config schema for the votingapi_widgets field was removed from the votingapi module. It never belonged in that module, but since votingapi is *always* installed when votingapi_widgets is used, that didn't cause any errors.

But now that it is gone from votingapi, we need to add it to votingapi_widgets where it belongs. And update it at the same time to correspond with the current state of this module.