Allow auto scrolling with middle button mouse click

beckyb

Allowing using the middle button on the mouse to start an auto scroll on the grid view. Some of the sheets are quite wide, and it would be nice to not have to use the scroll bar at the bottom to move left/right.

beckyb

Also, some of the sheets are quite long, and using the auto scroll feature can be faster than normal scrolling or using the scroll bar. :)