[kcmkwin/kwindecoration] Better presentation of tabs

Summary:
Tabs in the Window Decoration KCM look awkward now because the frame is drawing an extra line while the tab bar is also drawing its line.

This patch just slightly alters an existing hack so that the frame's top line can't be seen.

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D25582