[Accessibility] Accessibility tree Children are sorted in wrong order

Just like in QTBUG 75005, children of a control need to be sorted by y-axis in QAccessibleQuickItem before being used to navigate the tree. This is because child items get appended to the end of the children list of an item, whereas in the tree they should appear in the order they show up in the UI (top-down).

This is primarily a problem with ListView and anything that uses Repeater / Instantiator