[wayland] Fix some coding style issues in XdgShellClient

Summary:
The Frameworks coding style prohibits using short variable names, unless
it's a counter in a loop.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24198