screen: Fix for gnome-settings-daemon changes

g-s-d changed its DBus interface, and that made all
external changes to brightness invisible to the panel. Update
accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=662117

panels/screen/cc-screen-panel.c

@@ -186,21 +186,21 @@ on_signal (GDBusProxy *proxy,
 {
   CcScreenPanel *self = CC_SCREEN_PANEL (user_data);
 
-  if (g_strcmp0 (signal_name, "BrightnessChanged") == 0)
+  if (g_strcmp0 (signal_name, "Changed") == 0)
     {
-      guint brightness;
-      GtkRange *range;
-
       /* changed, but ignoring */
       if (self->priv->setting_brightness)
         return;
 
-      /* update the bar */
-      g_variant_get (parameters,
-                     "(u)",
-                     &brightness);
-      range = GTK_RANGE (WID ("screen_brightness_hscale"));
-      gtk_range_set_value (range, brightness);
+      /* retrieve the value again from g-s-d */
+      g_dbus_proxy_call (self->priv->proxy,
+                         "GetPercentage",
+                         NULL,
+                         G_DBUS_CALL_FLAGS_NONE,
+                         200, /* we don't want to randomly move the bar */
+                         self->priv->cancellable,
+                         get_brightness_cb,
+                         user_data);
     }
 }