Revert "Remove all uses of G_CONST_RETURN"

This reverts commit 36741245cc.

The removal has not been discussed, except on Bugzilla:

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

gobject/gvaluetypes.c

@@ -1073,7 +1073,7 @@ g_value_take_string (GValue *value,
  *
  * Returns: string content of @value
  */
-const gchar *
+G_CONST_RETURN gchar*
 g_value_get_string (const GValue *value)
 {
   g_return_val_if_fail (G_VALUE_HOLDS_STRING (value), NULL);