Fix progress size while printing

Summary:
Use QSpacerItem::changeSize() to fix the size when printing or not.
This Forces our Progress bar to use as much of the space as it can

Reviewers: tcanabrava, patrickelectric, laysrodrigues

Reviewed By: laysrodrigues

Subscribers: Atelier: AtCore

Tags: Atelier: AtCore

Differential Revision: https://phabricator.kde.org/D12267