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
This is a much better idea than what I did. I totally forgot about the description when I did the initial patch; it should probably work like the theme description. That  means we need to figure something else out for navigating back to the top-level; I'll look at some options for that.

I think rather than `ul.accordion-section-content`, which gets padding in css and wouldn't open like other accordions, we should probably use `ul.accordion-container.accordion-page-content`, making it the container for a separate accordion, and using the page-content part for the container that slides in on click of the top-level item (instead of the usual slide-down accordion behavior).

I think the page title should then be duplicated for display with the description in a `li.accordion-section`, with "you are editing..." and styled like the "you are previewing...".

Also, related to the js: https://core.trac.wordpress.org/ticket/23449#comment:6, haven't tried that but going to now.