Model panel: allow to actually select the Buhlman model: A, B, C

[eianlei]

At this time the model is always Buhlman C, although there is a widget to select any.
Just need to connect a handler for that.
Other:

• At the same go could implement a table view to see, and maybe even tweak the coefficients.
  • This allows exploring how the tweaking affect the calculation.
• Could also add widgets to edit the other basic parameters.
• A combo box to select the most common Gradient Factors
• Prepare to add selections for VPM-B and RGBM models too...