Added Scroll Bar to just watches

This addresses this issue: https://github.com/elm-lang/elm-reactor/issues/29

I've made changes so that this can be merged with the most recent changes.

This works by decoupling the Watches and Sliders into two distinct elm programs. Each one can be placed in their own div and communicate via ports. This allows us to add a scrolling area just for the Watches while leaving the Slider visible at all times.