Summary:
Use the KCM name in Qt Widgets KCM headers, to make them consistent with others.
{F6614892}

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