[about-distro] pimpl OSRelease privates
Summary:
in preparation of moving it into library
- private members are now in private class
- private class is dptr'd into public class
- public class has getters for all keys
- public class has docs
Test Plan: test still passes
Reviewers: apol
Reviewed By: apol
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D19975