[kcm] Expose interface to control output property retention

Summary:
This adds two radio buttons to the kcm, that allow to change the output
property retention of the output currently being modified.

The default value is global property retention.

{F6431307}

Closes T10028

Test Plan: Autotest passes.

Reviewers: #plasma

Subscribers: ngraham, plasma-devel

Tags: #plasma

Maniphest Tasks: T10028

Differential Revision: https://phabricator.kde.org/D16997