Debug scroll bar

To make this work, I decoupled the Watches and the Scroll Bar into two different elm programs each which uses the correct ports. I then updated core.js to use the correct handler.

This adds a scroll bar to the watches section such that you can scroll through them and still see the time travelling / swap options.

This resolves https://github.com/elm-lang/elm-reactor/issues/29. Tested in chrome and firefox.