Called if an object is about to be replaced. If both RhinoDoc.UndoActive() and RhinoDoc.RedoActive() return false, then immediately after the ReplaceObject event, there will be a DeleteObject event followed by an AddObject event. If either RhinoDoc.UndoActive() or RhinoDoc::RedoActive() return true, then immediately after the ReplaceObject event, there will be a DeleteObject event followed by an UndeleteObject event.