[kcmkwin/decorations] Use correct header text

Summary: The header text is supposed to be the KCM's name, not a duplicate of the tooltip text.

Test Plan: {F6615111, size=full}

Reviewers: #vdg, GB_2

Reviewed By: #vdg, GB_2

Subscribers: kwin

Tags: #kwin

Maniphest Tasks: T10273

Differential Revision: https://phabricator.kde.org/D19018