Include presenter notes when printing to PDF

[doersino]

This functionality is useful when distributing slides to attendees. It's implemented in most presentation software, e.g. in PowerPoint: https://www.cs.princeton.edu/~arvindn/talks/MIT-STS-AI-snakeoil.pdf

This would have to be a separate printing mode since printing the slides-and-nothing-else to PDF is a common use case. Not sure what the best way to trigger this would be. It could, of course, depend on a config option.