Gtk::Builder and item id

[Christian Schoenebeck <schoenebeck linuxsampler org>]

Hi

in the course of porting our app to latest GTKMM 3.91.2 API(s), I encountered 
some issues where I wonder whether those are GTK(MM) bugs or rather 
intentional.

First of all, I replaced Gtk::UIManager code occurrences by Gtk::Builder code. 
And for instance a popup menu which was previously constructed with UIManager 
like this:

    uiManager = Gtk::UIManager::create();
    uiManager->insert_action_group(actionGroup);
    Glib::ustring ui_info =
        "<ui>"
        "  <popup name='PopupMenuInsideDimRegion'>"
        "    <menuitem action='SplitDimZone'/>"
        "    <menuitem action='DeleteDimZone'/>"
        "  </popup>"
        "</ui>";
    uiManager->add_ui_from_string(ui_info);

    popup_menu_inside_dimregion = dynamic_cast<Gtk::Menu*>(
        uiManager->get_widget("/PopupMenuInsideDimRegion")
    );

with Builder it now looks like this:

    uiBuilder = Gtk::Builder::create();
    Glib::ustring ui_info =
        "<interface>"
        "  <menu id='menu-PopupMenuInsideDimRegion'>"
        "    <section>"
        "      <item id='item-split'>"
        "        <attribute name='label' translatable='yes'>Split Dimension 
Zone</attribute>"
        "        <attribute 
name='action'>PopupMenuInsideDimRegion.SplitDimZone</attribute>"
        "      </item>"
        "      <item id='item-delete'>"
        "        <attribute name='label' translatable='yes'>Delete Dimension 
Zone</attribute>"
        "        <attribute 
name='action'>PopupMenuInsideDimRegion.DeleteDimZone</attribute>"
        "      </item>"
        "    </section>"
        "  </menu>"
        "</interface>";
    uiBuilder->add_from_string(ui_info);

    popup_menu_inside_dimregion = new Gtk::Menu(
        Glib::RefPtr<Gio::Menu>::cast_dynamic(
            uiBuilder->get_object("menu-PopupMenuInsideDimRegion")
        )
    );

Now the problem is, that add_from_string() throws the following exception:

        Attribute "id" invalid for element "item"

Is that a bug? If that is intentional, how should it be possible to query an 
item object instead? Because we need to query menu items to change their label 
at runtime like this:

        auto menuItemSplit = Glib::RefPtr<Gio::MenuItem>::cast_dynamic(
                uiBuilder->get_object("item-split")
        );
        menuItemSplit->set_label(
                (bSomeCondition) ? _("Some text") : _("An alternative text")
        );

Thanks!

CU
Christian