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 really back and forth on this. While sections and subsections seems cleaner, having a distinct "page" concept seems easier to grasp and follow (explicitly adding a page, versus having to find if a section has children to determine its functionality when reading code). Should probably get feedback from a lead dev, since this is one of the most important parts. I think https://core.trac.wordpress.org/ticket/27406#comment:16 is a good explanation of the two approaches.