GtkButton: do not allow both types of actions

Only allow one of 'action-name' or 'related-action' to be set.

https://bugzilla.gnome.org/show_bug.cgi?id=667394

gtk/gtkbutton.c

@@ -732,6 +732,7 @@ gtk_button_set_action_name (GtkActionable *actionable,
   GtkButton *button = GTK_BUTTON (actionable);
 
   g_return_if_fail (GTK_IS_BUTTON (button));
+  g_return_if_fail (button->priv->action == NULL);
 
   if (g_strcmp0 (action_name, button->priv->action_name) != 0)
     {
@@ -1041,6 +1042,8 @@ gtk_button_set_related_action (GtkButton *button,
 {
   GtkButtonPrivate *priv = button->priv;
 
+  g_return_if_fail (button->priv->action_name == NULL);
+
   if (priv->action == action)
     return;