Extract Xauthority

Extract Xauthority from Xorg server and save it to user's home.
Fade in/fade out

When the user session is started a black overlay fades in and then
greeter's window is hidden so that users won't see it.
When the user session finishes, greeter's window is displayed and the
overlay fades out.
Send password for authentication

Instead of sharing the password input field we just set the password
after the user pressed ENTER key and after the authentication we clear
the property.
Spawn user session using QProcess

POSIX functions are cool but this work way much better.
Now we running user sessions works.