Better Screen PDFs

[bithappens]

At the moment the screen formats are only supported rudimentary. The following improvements seem reasonable

• Support other screen formats (portrait + landscape, other beamer formats, smartphones)
• Make beamer all files using the same key file as a reusable code base (also for other projects)
• Beamer slides should have full cells, but with the finish beyond the viewport of the slides so it only gets visible if multiple pages are printed at once