Reimplement Chart::paint() to fix printing issues

The previous implementation did not take the headers and footers
into account when resizing the diagram.

The implementation has been taken from AbstractArea::paintIntoRectArea().

This patch adds the method PrintingParameters::scaleFactor(), because
in branch 2.6 the class member PrintingParameters::scaleFactor was made
private.

BUG:405075
FIXED-IN:2.6.2

Test Plan: tested on linux
Reviewed By: danders
Differential Revision: https://phabricator.kde.org/D19541
(cherry picked from commit 347e5dda)