Adding custom color-fader widget.

Started adding color-editor screen.  Realized I needed
a fader widget -- I can't easily configure Apple's default
slider class to do it.

- Wrote fader widget:  it's really just a scrollbar.
- Wrote reusable class category for measuring views quickly.
- Made fader report its measured value to a delegate.
- Made the fader's custom "start" and "end" properties
  configurable from within Interface Builder using Apple's
  new ability to see your custom classes rendered in IB.
  For more info, see
  https://developer.apple.com/library/ios/recipes/xcode_help-IB_objects_media/chapters/CreatingaLiveViewofaCustomObject.html

- For now, my first prototype custom fader widget is just
  slapped in the middle of the home screen, so I can see
  and test it.