Finally implement proper randomization of option buttons
Move all manual color reduction code to Posterize tool

...and remove old posterize code.  This brings PD into line with other
modern photo editors.  My manual color reduction code is well-written
and unique, but it's impractical for converting to 8bpp, especially when
Xiaolin Wu and Neuquant methods are available.

As a custom posterize tool, however, it's a great better fit.  I like
this arrangement because PD now provides a more powerful posterize
interface than any other photo editor.