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
Ah, as you [commented](https://core.trac.wordpress.org/ticket/27406#comment:17) on the ticket:

> While pages and top-level sections could be combined in sorting with this approach, I kept them separate, meaning all pages would be displayed either above or below all top-level sections. I don't think we should enforce anything like requiring all sections to be in pages.