[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