[KConfigGui] Check font weight when clearing styleName property

Summary:
This is an addendum for commit a2774ff5b41987c3919a9e; making the check
tighter.

Test Plan: make && ctest

Reviewers: Frameworks, dfaure, davidedmundson, cfeck, ervin, meven, bport

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27811