[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.

Closes T10028

Test Plan: Autotest passes.

Reviewers: Plasma

Subscribers: ngraham, plasma-devel

Tags: Plasma

Maniphest Tasks: T10028

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