Enable the App to manipulate a GradientLayer

Seems to be a duplicate of #120 
GradientLayer (Bi-layer like)
- 2 independent and different outermost layers
- Results in N dependent (gradient) layers
- Thickness is given the N dependent layers
- Methods
  - Set number of gradient layers, Special
  - Set thickness of gradient layers, Special
- Uppermost and lowermost layer 
  - Set material
  - Set roughness
  - Constrain roughness