manager: add way for displays to register themselves
    
    The plan is to start the X server implicitly as part
    of starting the session. Once we do that, we'll need
    some way to communicate that the X server started
    successfully.
    
    This commit adds a RegisterX11Display method to GdmManager
    to handle that.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=744764