Mouse wheel events do not reach the interactor event mechanism.
In mitkEventTypeDef.h you find "typedef DisplayPositionEvent WheelEvent;".
This works because by now everyone uses the WheelEvent for clicks and the resulting position. Now we want additional information about the mouse wheel movement. So we should implement a new class mitkWheelEvent derived from DisplayPositionEvent which has an additional parameter for the mouse wheel movement (delta).
Also the QmitkEventAdapter class has to be modified to throw proper mouse wheel events.