Observing w and s char event

[paskino]

We observe w and s key events to auto window/level and save the view to png, but they are also observed by VTK to trigger wireframe or surface visualisation of polydata. This means that when we use them, both actions happen and especially switching rendering of surfaces is annoying.

We could consume w and s as I described in https://discourse.vtk.org/t/consume-a-vtk-event-in-python/13052/3