Editor variants and a string diagram variant for Petri nets

[kasbah]

closes #960
closes #955

hover:

[epatters]

This is great, thanks Kaspar! I like the overall conception for "editor variants," and the string-diagram-style editor variant Petri nets is very cool.

To my questions/comments below I would add that I agree with @tim-at-topos that the editor variants shouldn't appear in the list of logics/theories but should be settable via some other widget in the model notebook. But we don't have to figure that out right now.

[epatters]

These helpers are duplicative with the wrapping/unwrapping performed in src/model/object_input.

Behind this is the more important point that this component would ideally not bake in the assumption about a particular operation being applied (cf. similar comment below), which couples it unnecessarily closely to a particular theory. To be more specific, a Petri net can be viewed as free symmetric monoidal categories (SMCs) or free PROPs (or other things besides), and we might want to reuse this component for such situations. However, if it's not clear what to do, we can defer this to a future PR.

[kasbah]

I believe I've addressed all feedback and I've changed this to be behind a settings pane like the analysis settings.

out.mp4

[epatters]

Thanks, looks wonderful!