App to open PDF File from Settings

[dragotin]

In https://github.com/dragotin/kraft/blob/master/src/portal.cpp#L785 the code just calls QDesktopServices::openUrl(url) to open the PDF for printing.

It would be good if the code would check an entry in the kraft-settings for a viewer app, in case the xdg-open, which is called by QDesktopservices is failing for some reasons.