27406: Widget Customizer: Organize all widget area sections into a meta customizer section

https://core.trac.wordpress.org/ticket/27406

This PR is not to be merged. It is intended to provide a means of collaboration and code review. When work is complete, a patch file should be uploaded to the ticket, e.g. via https://github.com/x-team/wordpress-develop/compare/trac-27406.diff

/cc @celloexpressions
Yeah, although I suppose that was the idea with accordion-container. I don't think we could add a class there due to back-compat. But we could maybe use something other than accordion-container for a sub-container, if this is the first case of nesting accordions?