Fix menu item alignment with submenus

We were not aligning model button labels with nesting
submenus with indicators in their containing menu.

This was visible in the application demo of gtk4-demo.

gtk/gtkmenusectionbox.c

@@ -319,6 +319,7 @@ gtk_menu_section_box_insert_func (GtkMenuTrackerItem *item,
 
           widget = g_object_new (GTK_TYPE_MODEL_BUTTON,
                                  "popover", submenu,
+                                 "indicator-size-group", box->indicators,
                                  NULL);
           g_object_bind_property (item, "label", widget, "text", G_BINDING_SYNC_CREATE);
           g_object_bind_property (item, "icon", widget, "icon", G_BINDING_SYNC_CREATE);