Add support for Adobe Color Book (ACB) Palettes
This patch attempts to add support for loading Adobe Color Book (.acb) palettes.
These files are how Adobe traditionally stored its various Pantone, Trumatch, Toyo, and HKS palettes (8 bit colors for RGB/CMYK/Lab). It's also possible to create your own custom ones. The unofficial but detailed documentation for the format is here: https://magnetiq.ca/pages/acb-spec/. Sample ACB files can be found here: https://github.com/jacobbubu/acb/tree/master/color-books
If accepted, I think this would finish out the standard Adobe palette support in GIMP.
(@Wormnest Hello again! I think I applied everything we discussed in the ASE merge request, but if you have time and see anything I missed please let me know. Thanks!)