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
Looks like it doesn't actually work as an accordion when it's nested - `	$( '.accordion-container' ).on( 'click keydown', '.accordion-section-title', function( e ) {}` gets triggered twice in sub-levels. But other than that I almost have a new version that works with the nesting and has the sliding animation.