[platforms/wayland] Use xdg-shell stable protocol
We support the stable xdg-shell protocol nowadays. So use this one instead of
version 6 unstable.
Runs with other recent patches in KWin nested. In Weston nested session still
not yet working.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Differential Revision: https://phabricator.kde.org/D25129