QQuickItemPrivate::anchors creating anchors

Calling QQuickItemPrivate::anchors auto-creates the anchors, when not being allocated before.

This hurts as tools like GammaRay iterate over all properties to display their current value - accidentally changing the object tree + creating pointless classes + giving wrong results in this case.

In general I would recommend not offering any sort of property read methods with side effects - especially when being declared as const.