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