Changing the way that QmitkRenderwindow treats incoming Qt events...

Previous code broke betweeen MITK 2016.011 and 2018.04 due to
changes in VTK's QVTKOpenGLWidget. In 2016.11 QmitkRenderwindow
was checking all Qt events relevant for MITK's interaction
framework. QmitkRenderwindow decided whether the event shall
be treated by MITK or not - forwarding the event to VTK in the latter
case.

With changes to VTK's QVTKOpenGLWidget the above logic was not working
anymore. QVTKOpenGLWidget was getting to see relevant Qt events first,
e.g. mouse wheel events. Consequently, MITK interactors still got to
treat wheel events but could no longer prevent VTK from processing them
as well. That lead to behavior such as 'some MITK paint brush interactor
changes brush size' _and_ VTK's 3D camera interactor changes the zoom
level at the same time - wherase you would only one or the other
reaction.

The present change implements QmitkRenderwindow::event() which will now
get all Qt events first. QmitkRenderwindow::event() then decides on
further treatment as it was intended in the first place.

Signed-off-by: Daniel Maleike <daniel.maleike@stryker.com>