Initial support for latex beamer notes

I once wrote a patch for the original Pdf-Presenter-Console to support latex beamer notes on the second screen: jakobwesthoff/Pdf-Presenter-Console#44.

I've adjusted that patch to work with the current version of pdfpc.

The solution is quite simple. Latex renders notes at the right or left side, below or above the actual content slide. The patch simply splits the pdf into two parts and renders the notes in the presenter and the content in the presentation window. The position of the notes on the pdf is defined by a new parameter 'beamer-notes' or 'N'.