Track the lastCommand set to the printer

Summary:

• Track the last command sent to the printer to allow for command/ return pairing.
• Use this now power to fix a bug where any X could set posString that could later be problem is you paused a print.

Reviewers: laysrodrigues, patrickelectric, tcanabrava

Reviewed By: patrickelectric, tcanabrava

Subscribers: Atelier: AtCore

Tags: Atelier: AtCore

Differential Revision: https://phabricator.kde.org/D18468