Make QGraphicsView respect scrollbar policies

Make QGraphicsView respect scrollbar policies

When calculating the maximum height / width which a QGraphicsView can display,
make sure we only take the scrollbars' dimensions into account if their policy
is set to Qt::ScrollBarAsNeeded:

- if the policy is set to Qt::ScrollBarAlwaysOff, the scrollbar will not be
  displayed at all

- if the policy is set to Qt::ScrollBarAlwaysOn, the scrollbar's dimensions
have already been substracted from the available space by
QAbstractScrollArea::maximumViewportSize()

Task-number: QTBUG-14711
Change-Id: If5d24b41dbe7b089abca2bf61ccbd370d4de79a1
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>