Add a logging category for logs warnings

Summary: Disable logs unless the logging category kf5.kconfig.core is enabled

Test Plan: ctest

Reviewers: Frameworks, apol, cgiboudeaux

Reviewed By: cgiboudeaux

Subscribers: cgiboudeaux, mikeroyal, apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22061