xfce4session: make it systemd-aware

This should solve many issuses, including:
 - show mountable volumes and mount-on-click (in Thunar)
 - fix shutdown actions (and don't ask for password anymore)

It doesn't explicitly depend on anything in x-updates,
but AFAIK noone has tested it against master (yet).
Connected to #441.

pkgs/desktops/xfce/core/xfce4-dev-tools.nix

@@ -0,0 +1,24 @@
+{ stdenv, fetchurl, pkgconfig, glib, autoconf, automake, libtool, intltool }:
+
+stdenv.mkDerivation rec {
+  p_name  = "xfce4-dev-tools";
+  ver_maj = "4.10";
+  ver_min = "0";
+
+  src = fetchurl {
+    url = "mirror://xfce/src/xfce/${p_name}/${ver_maj}/${name}.tar.bz2";
+    sha256 = "1y1byfjciqhxqfxbjfp22bn5lxk3a01ng7zfjha8h5mzzfxlk5pp";
+  };
+  name = "${p_name}-${ver_maj}.${ver_min}";
+
+  buildInputs = [ pkgconfig glib ];
+
+  # not needed to build it but to use it
+  propagatedBuildInputs = [ autoconf automake libtool intltool ];
+
+  meta = {
+    homepage = http://foo-projects.org/~benny/projects/xfce4-dev-tools/;
+    description = "Tools and M4 macros for Xfce4 developers";
+    license = "GPLv2+";
+  };
+}