add mouse pushing that's not dependent on middle-button or wheel to slice3dVWR

[GoogleCodeExporter]

On machines without middle buttons or mousewheels (laptops for example), 
having a different way of pushing / pulling slices in the slice3dVWR would 
be useful.

This is related to issue 167.

Original issue reported on code.google.com by cpbotha on 1 Apr 2010 at 1:36