mouse wheel controls both slice position and zoom in the slice widget

[tlambert03]

would be great to have finer-tuned control over the exact roi being shown in the slice widget.

[maweigert]

Thanks. Somehow the wheelevent was still captured by the slider, which it should not. Fixed in develop now.

• mouse wheel now zooms into the slice
• left mouse pressed will move the displayed region
• left click on ("x","y","z") next to the slider cycles through the axis.