ScrollBar should allow separate step for buttons

[kevinfoley]

We have a ScrollBar for the user to scroll over a large area. In order to allow smooth scrolling by dragging the thumb with the mouse, the step has to be set to a very small value (e.g. 1). However, this means that scrolling with the increment/decrement buttons is so slow that they are totally useless.

We should be able to set the step used by the increment and decrement buttons separately from the step used for dragging the thumb with the mouse.