Give ScrollLayouts a "snap to" configuration parameter

[bendemboski]

This only makes sense if we implement #3. It we do implement #3 and have configurable intervals, then it would also be very useful to have a configuration parameter that instructs the scrollview to "snap to" the interval. In other words, rather than the observer (e.g. the hosting Dialog) knowing what the interval is and rounding down to the nearest interval as some do now, this would be encapsulated at a lower level.

This would probably involve the SliderContainer hiding this information -- all of the ScrollLayouts behave exactly as they do now, but the SliderContainer only notifies its observer when the time crosses an interval boundary, and then it sends the time of that boundary.

We might be able to get fancier and restrict the movement of the scrollers so they stick to their value until scrolled far enough, and then do a quick "bounce" to the next value.