[KColorUtils] Add hue(), chroma(), hcyColor() and update documentation

Summary:
This patch provides ways to get just the hue or just the chroma in the HCY colorspace and a way to get a QColor based on given hue, chroma, luma and alpha values.

This patch also adds more details to the documentation, such as the ranges of chroma and hue and the fact that KHCY hue, chroma and luma are computed in linear colorspace rather than sRGB.

Reviewers: Frameworks, dfaure, mwoehlke

Subscribers: mwoehlke, broulik, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27017