Using the extra window the ZoomControl keeps makes it impossible to
increase contentWidth to a point where it overflows. The drawback is that
the position on the scrollbar doesn't reflect the real position of the
visible part of the trace anymore if you zoom in to such a depth.

Task-number: QTCREATORBUG-11879
Change-Id: I6649f3c139f76c242a91d60364a28a4a00c9acee
Reviewed-by: Kai Koehne <kai.koehne@digia.com>