Add qauth support for x11-misc/sddm

Since https://github.com/sddm/sddm/pull/191 sddm depends on [qauth](https://github.com/MartinBriza/QAuth).

This PR provides a new ebuild for `sys-auth/qauth` and changes `x11-misc/sddm` to depend on it.
Although upstream's qauth support is still a bit broken, this makes sure everything's in place once it has been fixed.