Adapt to kscreenlocker interface v3
Interface v3 provides improved PAM support. This has to be reflected in GUI.
Continuation of !774 (closed) but I can't push to Aleksei's branch. My review comment wasn't acted upon so I'm following it up here. We do need to ensure backwards compatibility from screenlocker to themes, but not default theme to screenlocker. The author had things coded the wrong way round.