I think for it to work, it has to use [`gtk_menu_item_new_with_mnemonic`](https://developer.gnome.org/gtk3/stable/GtkMenuItem.html#gtk-menu-item-new-w...) [here](https://github.com/geany/geany/blob/1.30.1/src/tools.c#L564) instead of `gtk_menu_item_new_with_label`.