Draft: gdk/wayland: Implement toplevel tag protocol

The toplevel tag would an application developer to tag a toplevel surface with a unique per-app identifier.

When combined with the application-id, the compositor can use that information to identify specific windows and use it for configurability/scriptability

The protocol: https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/238

Note that we are planning to use an atom on X11 for backward compatibility as the majority of embedded apps are using X11 still but that shouldn't be very important for GTK.