Implement the "foreign" wayland protocol.
A client can export a surface with an unique string as handle,
then another client can refer to that surface and set an own surface as
child of that surface.
Potential use cases are out-of-process dialogs, such as file dialogs,
meant to be used by sandboxed processes that may not have the access
it needs to implement such dialogs.
The handle needs to be shared between the processes with other means,
such as dbus or command line paramenters.
The public api of the server side only tracks parent/child relationships as this is the only data kwin would need it for, the rest of the api is not exported so should be safer from eventual protocol changes