allow class parameter for hook_civicrm_links()

Use case:
Allow other modules to add non-civicrm links to "Actions" ribbon on Contact Summary

Currently you can add links however it tries to open it inside CiviCRM pop up hence it fails. Using class param (class=no-popup) you can control this behavior.
Note that class param is set only if passed in the hook, so this does not have any impact on existing implementation of this hook.