background: Fix memory leak in XML signal handling

When emitted in an idle, the item was correctly unref'ed, but not
when emitting the signal straight away.

https://bugzilla.gnome.org/show_bug.cgi?id=709243

panels/background/cc-background-xml.c

@@ -316,6 +316,7 @@ cc_background_xml_load_xml_internal (CcBackgroundXml *xml,
         emit_added_in_idle (xml, g_object_ref (item));
       else
         g_signal_emit (G_OBJECT (xml), signals[ADDED], 0, item);
+      g_object_unref (item);
       retval = TRUE;
     }
   }