Add password checking to screenlocker

A new Authenticator item is introduced and exported to QML. It provides
a slot tryAuthenticate taking the password as a string. If invoked it
creates a KCheckPass object which handles the communication with the
kcheckpass binary to verify the password. If the authentication succeeded
the Authenticator emits a succeeded() signal, on failure a failed()
signal.

The authentication process is bound to a grace timer. While this timer is
active it's not possible to authenticate again. This state is exposed
with a Q_PROPERTY and used by the UI to disable the password field and
unlock button.

REVIEW: 117321