Summary:
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

Test Plan:
the autotest works, but has a lot of random crashes when deleting surfaces,
unfortunately backtraces don't tell much and the crashes never occur when running into valgrind
behavior may still be wrong, depending on how the protocol is supposed
to work if more clients try to set the same exported surface as parent

Reviewers: #plasma, #kwin, davidedmundson, graesslin

Reviewed By: #plasma, #kwin, graesslin

Subscribers: davidedmundson, graesslin, plasma-devel, #frameworks

Tags: #frameworks, #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D7369