Comment 3 for bug 1288048

Relevant snippet from Matthias Clasen about how they do this in gtk.dialog:

 GtkWidget *label;

box = gtk_box_new (GTK_ORIENTATION_HORIZONTAL, 0);
gtk_widget_show (box);
gtk_widget_set_size_request (box, -1, 16);
label = gtk_label_new ("");
gtk_widget_set_margin_top (label, 6);
gtk_widget_set_margin_bottom (label, 6);
gtk_style_context_add_class (gtk_widget_get_style_context (label), "title");
gtk_box_set_center_widget (GTK_BOX (box), label);
 g_signal_connect_object (dialog, "notify::title", G_CALLBACK (update_title), label, 0);

gtk_window_set_titlebar (GTK_WINDOW (dialog), box);