Improve ksmserver debug output

Summary: Correct misleading debug output. Make all ksmserver output (apart from
the tests) controllable with QLoggingCategory.

Test Plan: Compiled, logged in and out both with & without
"ksmserver.debug=true" in qtlogging.ini

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D2935