Target items are now set from QQuickViewItem::prepareTransition()
instead of QQuickItemView and QQuickPositioner to ensure they are
for a displaced transition even if there is no matching target
transition.

Task-number: QTBUG-24535
Change-Id: I0a6c7e3c6198786527014d421b96fc562c6186dc
Reviewed-by: Martin Jones <martin.jones@nokia.com>