Currently mitkDisplayVectorInteractorScroll always increases / decreases teh currently visible slice by one.
This means that no matter how far you move the curser on the screen, the application will only go ONE slice further for each QEvent. This also causes EVERY slice to be rendered.

The combination makes scrolling through large datasets very slow.

mitkDisplayVectorInteractorScroll shall be extended to support faster scrolling through a dataset without having to render each slice.

The solution would be to introduce a modifier that defines how many pixels in displayCoordinates the mouse has to be moved to go one slice further.

A modifier of 4 (default) means that the user has to move the cursor by 4 pixels to go one slice further.
Moving the cursor by 20 pixels scrolls 5 slices further.

If the cursor has moved less than 'modifier', the application scrolls one slice only.