Update shader on demand

[bsrunnels]

The shade and select colormap buttons are not working correctly if the MD_object has already been loaded. Steps to reproduce:

1. Load an MD file
2. Open an ovito script and enable property calculation (e.g. centrosymmetry)
3. Reload file
4. Update "select shader" to specify "Centrosymmetry"
5. Click shade
   Result: the MD object is not shaded

Desired behavior would include:

• The "select shader" box should be a dropdown menu showing all available properties
• When the "select shader" box is updated, the "data min" and "data max" values should change accordingly
• When the shade button is clicked, update the texture.

I think this is the result of safeguards put in place to keep shaders from being overwritten. Let's change tactics instead and risk overwriting the existing shaders, and add safeguards later.