system-path, gnome3: run update-desktop-database to create the mime cache

This allows programs such as yelp to handle help:// protocol schemas

nixos/modules/config/system-path.nix

@@ -135,6 +135,10 @@ in
           if [ -x $out/bin/glib-compile-schemas -a -w $out/share/glib-2.0/schemas ]; then
               $out/bin/glib-compile-schemas $out/share/glib-2.0/schemas
           fi
+
+          if [ -x $out/bin/update-desktop-database -a -w $out/share/applications ]; then
+              $out/bin/update-desktop-database $out/share/applications
+          fi
         '';
     };
 

nixos/modules/services/x11/desktop-managers/gnome3.nix

@@ -75,6 +75,7 @@ in {
                                                 "${pkgs.glib_networking}/lib/gio/modules" ];
     environment.systemPackages =
       [ gnome3.dconf
+        pkgs.desktop_file_utils
         pkgs.glib_networking
         pkgs.ibus
         gnome3.gnome-backgrounds