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