Expose QQuickItem::mapRectToItem (and mapRectFromItem) to QML for better performance

QQuickItem::mapRectToItem and QQuickItem::mapRectFromItem should be exposed to QML in similar way as QQuickItem::mapToItem and QQuickItem::mapFromItem.

When working with e.g. 90 degree rotated views these mappings are required quite often, and it's pretty slow to do the mappings in the QML (or javascript) side. It's possible to create a C++ wrapper that simply calls these public functions, but for fully optimized QML version you'd probably need to implement a version that takes QQmlV8Function* as an input parameter.

(I reported this as a bug, and not as a suggestion, because this is a performance issue in my opinion.)