itegrate *splitslide* into notesslides.sty

[kohlhase]

Dennis made the splitslide and splitslidetikz environemnts for the AI lecture, and I have used it in various places and extended it in various ways and places. I think we should bite the bullet and make it official. Here are some things it still needs.

• We need to be able to pass on archive= information to tikzinput and mhgraphics. I guess we need an optional argument.
• This needs to also work in itemize (maybe even nested). I made a hacky isplit... environments in MathHub/courses/meta-inf/lib/course-preamble.tex This shows what we need, but needs to be generalized to multiple nesting depths.
• once this is in notesslides.sty all the definitions must be removed from the preambles.

[Jazzpirate]

I'm not convinced this belongs in notesslides.sty, actually. I'm happy with it staying in either a lib-package or a separate package, but I don't see it being related to semantic markup for slides...?

[Jazzpirate]

(It's also very much on the hacky-experimental-side of things that if integrated into notesslides would need a serious overhaul)

[kohlhase]

I am quite OK with putting it into a splitslide.sty but it is too userful not to work a bit more on it.