allow to set a custom palette instead of colorSets

Summary:
remove the public dependency to KConfigWidgets as
this is breaking existing apps, but instead let
applications set their own QPalette, so doesn't add
dependencies and is slightly more flexible

Test Plan:
old clients work, no new dependencies,
the kirigami custom palette icons is adaped and works

Reviewers: dfaure, Frameworks, rikmills, davidedmundson

Reviewed By: davidedmundson

Subscribers: davidedmundson, broulik, cfeck, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D8060