Completely rework Black and White (Grayscale) dialog

Now, all style options support variable # of gray shades and/or
dithering.  Much better!
Awesome dithering engine for Black and White dialog

Basically a copy/paste of the monochrome dithering engine, with some
minor adjustments as necessary.

With this rewrite, any grayscale algorithm can now output any number of
gray shades (2-256) while utilizing any known dithering option (10
choices available!).  Beautiful, fast results as well.

Not sure I could overload this dialog any more than I already have, but
it's fun to have options available.  Note that performance for regular
grayscale conversions is not impacted at all by this, as dithering will
be completely ignored if max # of shades are output (as is the default).