loginDialog: consolidate message label and login hint label

Right now the login hint is showing up just above the the cancel
button, instead of just below the text entry field.

The mockup here:

https://raw.github.com/gnome-design-team/gnome-mockups/master/system-lock-login-boot/login-dissect.png

Says it should share a label with the PAM info/error messages.

This commit consolidates the two labels.

https://bugzilla.gnome.org/show_bug.cgi?id=706324
Updated Slovenian translation