demo: ensure AdgGtkLayout is registered
demo: ensure AdgGtkLayout is registered

When a GtkBuilder file references a custom widget, you can potentially
get an "Invalid object type" error if that object has never been
registered before:

    https://stackoverflow.com/questions/44230814/invalid-object-type-when-using-a-custom-widget-in-gtk-builder-ui-file

Use `g_type_ensure` to avoid that error altogether.

ntd/adg,ntd/adg