...palette views despite selected color being in the currently
selected pallette

As suggested by Massimo, changing the color comparison EPSILON in
gimppalette.c from 1e-10 to 1e-6 fixes this, and is really small
enough.

Also, generally clean up color comparison epsilons:

- use a #define, not hardcoded values for all uses of
  gimp_rgb[a]_distance()
- call the #defines RGB_EPSILON and RGBA_EPSILON
- make them all 1e-6 or larger