make PointerHander.target a QObject; manipulate via metaproperties

If we allow adding handlers to 3D Model objects, and we want e.g. DragHandler to "magically" work the way it does in Qt Quick (just declare it and you can drag the object around), then DragHandler needs to learn how to drag 3D objects even though it's not linked against Qt Quick 3D. This could be done abstractly: QQuickItem is not the only kind of object that has x and y properties, but those are the only properties that DragHandler needs to set. (Currently it calls setPosition() though.)

WheelHandler sets metaproperties rather than calling setters directly. This was done to keep interceptors (like Behavior and BoundaryRule) working; and that is another reason that every handler that sets properties on items should do it via metaproperties. So I think there's a chance to solve two problems at once.