Bug 783680 - Allow to hide color models in GimpColorScales

Redo the entire thing again:

- Rename the values of enum GimpColorSelectorModel to include "MODEL"
- Change GimpColorSelector API from set_model() to set_model_visible()
  so visibility of each model can be toggled individually and is not
  exclusive any longer
- The GUI is back to what it was before, except that the "GIMP" page
  now honors the model visibility and has a resonable minimum height