fullJSON access to computed values #5185

We need to expose computed layout.margin values in fullJSON somewhere, as well as the computed tickangle and other computed automargin-impacting values.

I propose that we either add a new "read only" layout.computed attribute that has sub-attributes margin.{r,t,l,b} and x[2,3,4].{tickangle, etc}.

Alternatively, we could overwrite these when doing the fullJSON export in downloadImage so these values are available via Python's new full_figure_for_development mechanism without new attributes.