be able to use pointer handlers directly on 3D Model objects

It seems to be quite doable. in spite of handlers having been designed for 2D. We can add 3D coordinates to QQuickHandlerPoint, and have them populated only when relevant: that's a way to expose 3D QEventPoint positions to QML.

QQuickPointerHandler's target and parent properties are both QQuickItem* though. Code that assumes it will be of that type and not null needs work.

A harder problem is getting handlers' built-in behavior working: e.g. DragHandler only knows how to drag an Item. Dragging other kinds of objects can be done via bindings.