QML ItemViews inside QML ItemViews create all the items of its height on creation

If you run the attached listviewdelegatecreation_qt.qml file on qmlscene you'll see that the 50 items of the first section of the list are created even if just 6 are on screen, this is because itemviews create items based on their contentY and height, which in case of a itemview inside a itemview is "all the height", so all items are created.

I am proposing a new API to itemviews (will post the gerrit url in a comment) that allows users specify a range where delegate creation should happen, this is used in listviewdelegatecreation.qml to show that instead of the 50 items we just need to create 10 items (6 visible + cache)