Set the color scheme to Printing for Print Preview

Summary:
Otherwise the print preview dialog somehow gets the text color from the
current default color scheme; also the user can't change the color scheme
either from the print preview dialog or the "print" dialog launched from
the preview dialog.

BUG: 391678
FIXED-IN: 5.55.0

Test Plan:
- Change the default color scheme in kate to a dark one, e.g. Breeze dark
- Open the print preview dialog, note that the text color is the one used
  in the breeze dark scheme, but the background color is still white

Apply the diff, now the print preview dialog should be using the Printing
color scheme as expected.

Reviewers: cullmann, #ktexteditor, dhaumann, mwolff

Reviewed By: #ktexteditor, dhaumann, mwolff

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: #kate, #frameworks

Differential Revision: https://phabricator.kde.org/D18163