expose the colorset for the stylesheet

Summary:
this makes possible having icons loaded with different
colors in different parts of the application, will be needed
by kirigami (which will use this with a plasma-integration extension)

Test Plan: i can have icons colored in different ways across a single app

Reviewers: Frameworks, Plasma, hein, dfaure

Reviewed By: dfaure

Subscribers: plasma-devel, Frameworks

Tags: Frameworks, Plasma

Differential Revision: https://phabricator.kde.org/D7891