Fix print preview in Kig

Summary:
Currently, print preview shows nothing due to the lack of connection.

This patch tries to fix this.

Test Plan:

1. Open an example plot.
2. Select "File -> Print preview". The print preview should be shown.

Reviewers: KDE Edu

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D17585