Making the Kelvin picker work.

Got the slider to slide, and report a percentage change.  Not yet reporting the actual Kelvin value.

Also simplified the UI structure.  I was trying to do things the way I thought was Apple's "right" way, but it was adding a bunch of complexity, and it was hard to maintain.  The UI controls in the WNC color controller are actually in that controller, now, instead of nested and dereference 3 layers away.