can't use event handlers as children of View3D

It's quite natural to use TapHandler or HoverHandler for picking, instead of MouseArea. But QQuick3DViewport has Q_CLASSINFO("DefaultProperty", "data"), overriding the data property in QQuickItem, which breaks some of the "magic" in
QQuickItemPrivate::data_append(). It would be best to reuse that code; but if that's not practical for some reason, more of it needs to be duplicated, because it's not only items that can be children of items.