greeter: add small delay when presenting messages
    
    When PAM sends a message up to the greeter to show the user,
    it shows it right away, immediately overwriting any previous
    message.
    
    This commit introduces a message queue, so that each pending
    message gets a reasonable amount of time on screen for the user
    to read.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=583856