Improve Shadow Sandcastle UI

[lilleyse]

For #2594

Since the Shadows sandcastle is getting more complicated, there are a few simple UI tweaks to make it easier to use.

Certain UI elements can be grayed out when not applicable. For each of the light types, gray out the following properties:

• Freeform:
  • Radius
• Sun:
  • Light Horizon
  • Light Angle
  • Radius
• Fixed:
  • Light Horizon
  • Light Angle
  • Distance Limit
  • Radius
  • Cascades
  • Cascade Colors
  • Fit near/far
• Point:
  • Light Horizon
  • Light Angle
  • Distance Limit
  • Cascades
  • Cascade Colors
  • Fit near/far
  • Soft Shadows (may be supported in the future though)

@lasalvavida Feel free to take this when you are ready. Make a pull request into the shadows branch.

[lilleyse]

In another PR, we could give a dropdown so the user can chose which models are added to the scene.

[lilleyse]

May as well keep this open for future UI stuff.

Some other Sandcastle improvements that would be nice are:

• Dropdown to load different models. There are three main models that have been useful for testing for far (with more to come)
  • Wood_Tower.gltf
  • Shadow_Tester.gltf
  • Shadow_Tester_Point.gltf
• Dropdown to chose from different locations, mainly to test terrain shadows. I've been looking at these three
  • Everest
  • Exton
  • Pinnacle PA