Import Candela license & citations on WRX import

Per the request for more granular pull requests :)

This set of changes just adds import of Candela license and citation metadata on WRX import.

The approach has been rewritten to be much cleaner too.  The class-pb-wrx file was modified to add a new filter hook for meta fields to update from import.  The Candela citation and license plugins then use this new filter hook to register their meta fields for update on import.