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