Use sceneToWorld helper function here
Insert objects in center of window for Viewport
Grab items on insertion into a Viewport
Fix sneaky bug in ConeControl construction.

Because we're building it on top of a CylinderControl, we need to delete
the _CylinderRadiusControl that's constructed (and replace it with a
_ConeRadiusControl).  However, this causes trouble when the system
tries to make ControlProxies for the soon-to-be-deleted object.