Fix tooltip positioning

The previous fix worked only properly when the QQuickWidget was located in
the top-left corner of the window. With bigger offsets the tooltips were off.

Task-number: QTBUG-39878
Change-Id: Ia25194175aa9f9daa9c7132fc776d4b2fa874c26
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>