WIP Use QAuth instead of Authenticator and Session classes

Work in progress: Initial port to QAuth. Please review and comment.

Why?
* Handles PAM right
* Asynchronous
* Automagic
* Handles more use-cases than the original implementation did
* Reusability

What was not done (yet) on the SDDM side:
Doesn't use the asynchronous API as it should
Doesn't handle the password change dialog

TODO: massive architectural changes to both the daemon and the greeter