Make SerialLayer completely private

Summary:
Serial Layer shoudn't be exposed to anything higher then atcore.
Clients need to reconnect their logWidget to Atcore::pushedCommand
Should land before D16181

Reviewers: patrickelectric, laysrodrigues, tcanabrava

Reviewed By: laysrodrigues, tcanabrava

Subscribers: Atelier: AtCore

Tags: Atelier: AtCore

Differential Revision: https://phabricator.kde.org/D16199