feat: read/write output control files

Summary:
Adds functionality to read and write the control files of outputs. This can be
used going on for storing output-specific control info.

Test Plan: Compiles. Retention changes are still possible.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26030