mail_filter operators domain and localpart

Commit d907c0ce with the comment 'Fix for gentoo' includes an unrelated change to interface/lib/plugins/mail_user_filter_plugin.inc.php

This introduces additional mail_filter operators domain and localpart for which I see no frontend code.

@till Any idea if that's still dangling in an issue somewhere?

Would be nice to have that available here and in roundcube.