element: add method to get metadata

Add a method to get the metadata from a klass.
diff --git a/gst/gstelement.h b/gst/gstelement.h
index 28b23ec..7cf8fda 100644
--- a/gst/gstelement.h
+++ b/gst/gstelement.h
@@ -674,6 +674,8 @@
                                                                  const gchar     *author);
 void                    gst_element_class_add_metadata          (GstElementClass * klass,
                                                                  const gchar * key, const gchar * value);
+G_CONST_RETURN gchar *  gst_element_class_get_metadata          (GstElementClass * klass,
+                                                                 const gchar * key);
 
 
 /* element instance */