New port: graphics/gcolor3

Gcolor3 enables you to pick the color from any pixel on your screen.
It also offers a palette, so that you can easily mix and match a
couple of colors together.

When you have found the perfect combination of colors, naturally you
want to save them. Gcolor3 allows you to conveniently save and
retrieve colors.

WWW: https://hjdskes.github.io/projects/gcolor3/

Approved by: lme (mentor)
Differential Revision: https://reviews.freebsd.org/D10481