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 did a full patch implementing it the alternate way (without pages, just sections with parents/children), and left some comments on the ticket: https://core.trac.wordpress.org/ticket/27406#comment:23