Add capability to nest multiple levels of collapsible panes inside one another

[zacharysawyer]

Currently the new collapsible panes can only contain PlotInfos, but it would be really useful to be able to have mulitple levels of collapsible panes within collapsible panes.

[ulmangt]

With version 1.3.1 it's now possible to nest GroupInfo plots. However, there's a bit of work still to be done. Mainly, there's no indentation of sub-plots, so it's difficult to see the nesting structure.