remove QtWidgets dependency

The module doesn't depend on QtWidgets any more.

Change-Id: Ic8036ec1ee53ef8b1a3bb6c827fb815f322865a2
Reviewed-by: Martin Gräßlin <mgraesslin@kde.org>
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>