cups-pk-helper: init at 0.2.5

Adding this package to environment.systemPackages stops the
"Add new printer" button in gnome-control-center from being grayed out
and stops it from printing:

(gnome-control-center:16664): printers-cc-panel-WARNING **: Your system does not have the cups-pk-helper's policy "org.opensuse.cupspkhelper.mechanism.all-edit" installed. Please check your installation

But completing the printer setup requires some additional packaging
work. This is what happens when trying to _add_ a printer:

(gnome-control-center:18733): printers-cc-panel-WARNING **: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.fedoraproject.Config.Printing was not provided by any .service files
(gnome-control-center:18733): printers-cc-panel-WARNING **: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.PackageKit was not provided by any .service files

pkgs/misc/cups/cups-pk-helper.nix

···
+{ stdenv, fetchurl, intltool, pkgconfig, glib, polkit, cups }:
+
+stdenv.mkDerivation rec {
+  version = "0.2.5";
+  name = "cups-pk-helper-${version}";
+
+  src = fetchurl {
+    url = "http://www.freedesktop.org/software/cups-pk-helper/releases/cups-pk-helper-${version}.tar.xz";
+    sha256 = "0651ij5p5s0n3xxbaqsy72s22nx9hfkrjgvg766lkqd1cpniw8hr";
+  };
+
+  buildInputs = [ intltool pkgconfig glib polkit cups ];
+
+  meta = with stdenv.lib; {
+    description = "PolicyKit helper to configure cups with fine-grained privileges";
+    homepage = http://www.freedesktop.org/wiki/Software/cups-pk-helper/;
+    license = licenses.gpl2;
+    platforms = platforms.linux;
+    maintainers = [ maintainers.bjornfor ];
+  };
+}

pkgs/top-level/all-packages.nix

···
 
   cups_filters = callPackage ../misc/cups/filters.nix { };
 
+  cups-pk-helper = callPackage ../misc/cups/cups-pk-helper.nix { };
+
   crashplan = callPackage ../applications/backup/crashplan { };
 
   gutenprint = callPackage ../misc/drivers/gutenprint { };