Added gnome-vfs authentication dialog support.
        Currently works with HTTP basic and proxy authentication.

        A bug causes the dialog to appear more than once in quick succession
        in some cases.

        Separate processes (eg sidebar components) that need to access
        a protected URL will also pop open their own individual dialogs
        at this point

        There's also no integration with Mozilla at this point, so
        Mozilla too will prompt separately.

        This code works against gnome-vfs head and the gnome-vfs-1-0
        branch, as well as the soon-to-be-released gnome-vfs 1.0.2