Webform submissions data views relationship handler can't be easily altered

First off, thanks for #680386: Views integration for the webform_submitted_data table, whohoo! That's just awesome!

Secondly, the relationship handler defined is a bit difficult to alter as it completely overwrites the $this->definition['extra'] array rather than just adding new elements to it.

This means for my custom relationship handler, which extends webform_handler_relationship_submission_data, I can't add anything to that array - which I want to do so I can filter multi-value fields by the delta / data entered.

The attached patch fixes this so it plays likely with other relationship handlers which extend this one.