[platforms/wayland] Use xdg-shell stable protocol

Summary:
We support the stable xdg-shell protocol nowadays. So use this one instead of
version 6 unstable.

Test Plan:
Runs with other recent patches in KWin nested. In Weston nested session still
not yet working.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25129