Add support for logging categories

Summary:
Adds category logging to the KScreenLocker daemon/library and the greeter.

Replaces all current uncategorized messages with categorized ones in these
parts.

Test Plan: Tested messages with autotests and QT_LOGGING_RULES="kscreenlocker*=true".

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28389