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 think the customizer accordion should be cleaned up to support nested sections, so that the sections within a page can be output as child elements of the `accordion-section-content`, i.e.:

```php
/**
 * Render the page, and the sections that have been added to it.
 *
 * @since 4.0.0
 */
protected function render() {
	?>
	<li id="accordion-section-<?php echo esc_attr( $this->id ); ?>" class="control-section control-page accordion-section">
		<span class="preview-notice"><?php _e( 'You are editing your site&#8217;s' ); ?></span>
		<h3 class="accordion-section-title" tabindex="0"><?php echo esc_html( $this->title ); ?></h3>
		<ul class="accordion-section-content">
			<?php if ( ! empty( $this->description ) ) : ?>
				<li><p class="description"><?php echo $this->description; ?></p></li>
			<?php endif; ?>
			<?php
			foreach ( $this->sections as $section ) {
				$section->maybe_render();
			}
			?>
		</ul>
	</li>
	<?php
}
```

This would make it much easier to show/hide the pages since they'd all be logically contained within a common parent `ul` element, following the page's description if supplied.