Add method to write retention to configuration control files

Summary:
This patch adds a small method to ControlConfig which writes modified
retention values to the control file of the configuration.

Test Plan: Compiles.

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: mart, plasma-devel

Tags: #plasma

Maniphest Tasks: T10028

Differential Revision: https://phabricator.kde.org/D16996