We historically have both the colormap and palette concept in core GIMP,
though they are actually kinda the same concept, except that with
"colormap" we work with an array of raw data and it's a lot less
color-aware. It is still much more efficient in some specific cases,
such as when converting the image (we can then convert the whole palette
as a single buffer, because the image palette is space-restricted
anyway), when storing undo data or when storing/loading in XCF.

But for all the rest, let's use gimp_image_get_colormap_palette() and
work with the GimpPalette API.