Define ShellClient debug output

Summary:
Add a debug format for ShellClients, otherwise the output is always
empty. Also add the nospace qualifier to ShellClient's and Client's
debug output to present it correctly.

Test Plan: Manually.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15560