Remove all uses of G_CONST_RETURN

Just use 'const'.

https://bugzilla.gnome.org/show_bug.cgi?id=644611

gobject/gsignal.c

@@ -1223,7 +1223,7 @@ g_signal_list_ids (GType  itype,
  *
  * Returns: the signal name, or %NULL if the signal number was invalid.
  */
-G_CONST_RETURN gchar*
+const gchar *
 g_signal_name (guint signal_id)
 {
   SignalNode *node;