This patch attempts to provide a solution to the first issue noted in #7899 (closed). It converts the CMYK Color Selector to use babl for color conversions rather than using GimpColorTransform.
It also resolves an issue noted in this comment: When a CMYK profile is assigned, the color values are accidentally divided by 100.0 twice (so 100 becomes 1). The GUI spin scales move in steps of 1 and have a range of 0 - 100. Therefore, it is currently not possible to manually adjust the CMYK values with the sliders if a CMYK color profile is set. This patch resolves the issue as part of the conversion to babl.
Note that unlike !589 (merged), this patch does not add support for using the soft-proofing profiles in the View menu. I thought it would be better to submit the self-contained babl conversion first, then follow up with the soft-proofing profile changes once a decision has been made about how to handle them.
Now we also use the Image's soft-proofing color profile, and reach to changes accordingly.