Add QAuth library and helper

Currently SDDM provides only a simple username/password authentication.
More exciting credential requests such as password change are not supported.

PAM also requires double forking to work better and that's not currently
implemented.

Importing QAuth here opens a lot of possibilities.

More changes to SDDM are required to handle more complicated credential
requests, for now we just want to start using QAuth to gain the same
features as current SDDM. We will improve as we move forward.

More details: https://github.com/sddm/sddm/wiki/Authentication
Use QAuth for authentication

Now that SDDM includes QAuth we can take advantage of its improved
authentication support.

This is based on PR #155 by Martin Bříza <mbriza@redhat.com>
Improve greeter stderr and stdout logging from daemon

Unquote output and avoid extra spaces.
Use sddm PAM service in QAuth's PAM backend

Use our naming for PAM services.
Keep in mind that we don't have the sddm-check PAM service yet.
Use message handler for sddm-helper

Log just like sddm-daemon and sddm-greeter.
Also remove redundant parts from the log messages.
Merge pull request #191 from plfiorini/qauth

Improved authentication mechanism with QAuth