Start of port to Qt5

There are some things that need to be done before this can be merged:

 - Get a build of Qt5 that contains the `QWebPrinter` class (other than that this should work)
 - Test it vigorously
 - Cleanup the code

If someone with more knowledge of wkhtml2pdf (and a Qt5+`QWebPrinter` build) could finish this off, that would be great.