Use KCM name in KCM header

Summary:
Use the KCM name in Qt Widgets KCM headers, to make them consistent with others.

Test Plan: Open a Qt Widgets KCM like "Widget Style".

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, plasma-devel, VDG, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19006