vtkInteractorStyle already forwards StartInteraction and EndInteraction
events via the vtkRenderWindowInteractor set on it. It only makes sense
to forward the InteractionEvent as well. Applications needing
StartInteraction and EndInteraction events are quite likely to need the
InteractionEvent as well. Current approach is to list to that event
directly on the vtkInteractorStyle instance -- which is a little weird
since the StartInteraction/EndInteraction events can be directly
observed on the vtkRenderWindowInteractor.

Change-Id: I61d6ee39b53f2b990d2a1aad5df7bcf18d3800e1