attempt to work around Qt printing bugs

NOTE: Saving & loading the margins works around QPrinter/QPrintDialog bugs:
- https://bugreports.qt.nokia.com/browse/QTBUG-15351
- https://bugs.kde.org/show_bug.cgi?id=205802
- https://bugs.kde.org/show_bug.cgi?id=180051
Changing the margins now works. However, when you reopen the print dialog
later, the WRONG margins are displayed. The correct ones are still used.
This is a critical bug in Qt.

The real issue is Qt. And it's a critical one. But since it wasn't fixed for
years, it probably will never get fixed. That's why this workaround is better
than nothing...