Make it possible to change icon themes from elsewhere in kns
Summary:
Splits the logic into a separate binary, also cleans some unneeded code in
the KCM.
Introduces the experimental AdoptionCommand
Test Plan: Did so, been playing around with it.
Reviewers: Plasma, mart
Reviewed By: mart
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D4317