Add xdg-shell to stable/

Add a copy of xdg-shell unstable v6 to stable/xdg-shell/xdg-shell.xml.
Folliwing this commit, it will go through a set of changes, before
being declared stable.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: David Edmundson <davidedmundson@kde.org>