Provide option to persist auto-grouping

[hahnjo]

The auto-grouping feature is very handy for overlays created by latex-beamer. However, it doesn't work with show notes on second screen because that messes up page numbers in the PDF and the --notes option to pdfpc automatically disables auto-grouping (probably for that very same reason). What I would like to do is compile my slides first without notes, somehow persist the auto-grouping into a .pdfpc, and finally compile with notes and use the persisted grouping.

Right now, there are two complications to this:

1. pdfpc only writes pages that have a change by the user: either manually defined as an overlay, or an added note. Because the purpose is to use the auto-grouping for overlays, I currently go through all slides and enter an empty note.
2. The resulting .pdfpc has all the data I need, but without forcedOverlay and my understanding is that the information is still ignored as soon as I pass the --notes option. So I just replaced "overlay":[1-9] by \0,"forcedOverlay":1 and this seems to work.

My plan would be to add an option to perform these steps in pdfpc. Does this make sense and would it be accepted?

[fnevgeny]

IMO, there should be a limit of extra miles to which one software goes introducing dirty workarounds for bugs in another software. Especially when the latter is free/open-source. If you have time and incentive, perhaps you'd rather try fixing the issue with latex-beamer?...

[fnevgeny]

Please have a look at #615