More ways to control the slice index?

Possible ways to control the slice index:

  * [x] A slider.
  * [x] Clicking in one view sets position of other slicers of the same scene.
  * [ ] Scrolling with modifier?
  * [ ] Keyboard shortcuts?
  * [ ] Next/Previous buttons? -> Can be added by user if needed. We have an example that does this.
  * [ ] Numeric input? -> tricky because of recursion. Not so much needed?
