treeview: Use gtk_widget_set_overflow()

We need to clip again so we don't overdraw the headers, but setting
overflow does the right thing with header widgetry.

gtk/gtktreeview.c

@@ -1654,6 +1654,7 @@ gtk_tree_view_init (GtkTreeView *tree_view)
   priv = tree_view->priv = gtk_tree_view_get_instance_private (tree_view);
 
   gtk_widget_set_can_focus (GTK_WIDGET (tree_view), TRUE);
+  gtk_widget_set_overflow (GTK_WIDGET (tree_view), GTK_OVERFLOW_HIDDEN);
 
   priv->show_expanders = TRUE;
   priv->draw_keyfocus = TRUE;