Export Plasma core lib log category, add a category to a qWarning

Test Plan: Builds, new context for warning is visible

Reviewers: Frameworks, broulik, davidedmundson, apol

Reviewed By: davidedmundson, apol

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23266