Allow to print pdf doc directly into a QPrinter

This is the 'official' Qt way of printing. It avoids the conversion of pdf files to postscript that we do currently, and may fix a number of printer settings bugs.

There is a big caveat to this approach, however: Rendering a pdf doc into a QPrinter object only works with the poppler 'QPainter' backend. While that backend works okay on many documents, it is still lacking important features, and you may see rendering problems. For the most part this is not a fundamental problem. Somebody just needs to sit down and write the missing parts.

For this reason, CUPS printing remains the default, and Qt printing is simply a new option. Additionally, I tagged this option as 'experimental' in the GUI.