[KColorUtils] Change getHcy() hue range to [0.0, 1.0)

Summary:
As suggested by @mwoehlke in D27017

Users are likey to expect a value from 0.0 to almost 1.0 rather than [-1/6, 5/6)

Reviewers: Frameworks, dfaure, mwoehlke

Subscribers: kde-frameworks-devel, mwoehlke

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27192