Fixed BUG #12723. Add new mechanism to register event ids.
vtkCommand::EventIds were prone to mismatch. Developers ended up updated the header without updating the cxx file more often than realized and the resulting bugs were not obvious. Changing the mechanism to register the event ids so that such errors don't happen in the future. Change-Id: I1087cf1bce9e7e22b033875d645a4b2378bf8362