gtk3 menubutton type header bar - gtk3

Question

I googled this question but haven't found a solution. I'm transitioning from gtk2 to gtk3. I like the way how the menu icons are packed on the header bar. I was able to add a menu button to my header bar but it shows a down arrow button. Is there a way of changing this to a icon that has 3 lines.
This is exactly how I want it to be:
My present code looks like this
header_bar = gtk_header_bar_new();
gtk_header_bar_set_show_close_button (GTK_HEADER_BAR (header_bar), TRUE);
gtk_header_bar_set_title(GTK_HEADER_BAR(header_bar),PRG_NAME);
gtk_header_bar_set_subtitle(GTK_HEADER_BAR(header_bar),tool_version);
//gtk_header_bar_set_decoration_layout(GTK_HEADER_BAR(header_bar), "menu,minimize,maximize,close");
gtk_window_set_titlebar (GTK_WINDOW (window), header_bar);
//menu_popover = gtk_menu_new();
menubar = gtk_menu_button_new();
gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar),menubar);

Answer 1

If you use a XML file to store the ui, you can customize the icon like this:
<object class="GtkMenuButton" id="button_header_menu">
<property name="image">image_header_menu</property>
</object>
<object class="GtkImage" id="image_header_menu">
<property name="visible">True</property>
<property name="icon_name">open-menu-symbolic</property>
</object>
if not, you can do it this way:
icon = Gio.ThemedIcon(name="open-menu-symbolic")
image = Gtk.Image.new_from_gicon(icon, Gtk.IconSize.BUTTON)
button_header_menu.get_child().destroy()
button_header_menu.add(image)
button_header_menu.show_all()
I code in python, but it should be pretty straightforward to port this bit of code.

Answer 2

Thanks for putting me on the right track. below is how i did it
header_bar = gtk_header_bar_new();
gtk_header_bar_set_show_close_button (GTK_HEADER_BAR (header_bar), TRUE);
gtk_header_bar_set_title(GTK_HEADER_BAR(header_bar),PRG_NAME);
gtk_header_bar_set_subtitle(GTK_HEADER_BAR(header_bar),tool_version);
//gtk_header_bar_set_decoration_layout(GTK_HEADER_BAR(header_bar), "menu,minimize,maximize,close");
gtk_window_set_titlebar (GTK_WINDOW (window), header_bar);
//menu_popover = gtk_menu_new();
menu = gtk_button_new_from_icon_name("open-menu-symbolic", GTK_ICON_SIZE_BUTTON);
gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar),menu);