gtk_box_pack_start (GTK_BOX (vbox), hbox, FALSE, FALSE, 0);
button = gtk_button_new_with_mnemonic ("_Add new Column");
g_signal_connect (button, "clicked", G_CALLBACK (add_clicked), left_tree_model);
gtk_box_pack_start (GTK_BOX (vbox), hbox, FALSE, FALSE, 0);
button = gtk_button_new_with_mnemonic ("_Add new Column");
g_signal_connect (button, "clicked", G_CALLBACK (add_clicked), left_tree_model);