QQuickWindow: Consider z-order of children when delivering pointer events

When creating a target item list for handling pointer events, put children after the parent item if they have negative z-order value. This fixes an issue where an item does not receive a pointer event if there is a child item that accepts the event even when that child item is shown under the parent item as per the stacking order.

https://codereview.qt-project.org/c/qt/qtdeclarative/+/288660