Sorting itemsToPolish in top down order

[uwerat]

QQuickWindow calls QQuickItem::updatePolish in order of when QQuickItem::polish has been called ( = more or less random ). As one of the main ideas behind polishing is to calculate layouts, calling updatePolish in top down order is way more efficient.

To be able to reorder the QQuickWindowPrivate::itemsToPolish vector in O(n log n) QskControl needs to maintain its depth inside the QuickItem tree. Something that needs to be done on QQuickItem::ItemParentHasChanged - similar to what is already done for the locale.