gtk_box_pack_start (GTK_BOX (box), button, TRUE, TRUE, 0);
g_signal_connect (button, "value-changed", G_CALLBACK (on_opacity_changed), app);
gtk_box_pack_start (GTK_BOX (box), button, TRUE, TRUE, 0);
g_signal_connect (button, "value-changed", G_CALLBACK (on_opacity_changed), app);