The Wayland implementation of `GdkToplevel`.
Beyond the [iface@Gdk.Toplevel] API, the Wayland implementation has API to set up cross-process parent-child relationships between surfaces with [method@GdkWayland.WaylandToplevel.export_handle] and [method@GdkWayland.WaylandToplevel.set_transient_for_exported].