Fix mapping from slider values to menu items.

[garthk]

Distributes inputs in the range 0..100 evenly across the menu items, fixing a bug making it almost impossible to select the first item.

I'm happy with the logic. I'm sure there's a shorter, clearer version of the expression in slider_zone. I'm very sure the slider_zone function belongs elsewhere; @geekscape do you want to move it yourself afterward, or should I move it now?