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
> I'm starting to think that maybe pages should support controls as direct children. An example use-case would be the add-menu interface for the Menu Customizer, which will allow users to create new sections. That being said, I wouldn't want to support pages that acted as containers only for controls, not sub-sections.

@celloexpressions so then why not just make everything a `WP_Customize_Section`, but add support for a `parent` arg so that they can be nested? There would also need to be a `WP_Customize_Section::$subsections` array.