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Export: Print and PDF-export respect document font

When using Highlight2Html class for printing the font wasn't respected anymore.
Concretely the <pre> tag caused some different setting to be used.
Actually the font isn't used at all in HTML export, which is generally good. But maybe we add an option for that (which would then be auto-set for printing)

The (slightly hacky?) solution is to conditionally inject an explicit <pre> CSS class when exporting to PDF or printer.