Customizer: Improve ability to filter active state for widget area Customizer sections.

* Mark panels, sections, controls as active if preview explicitly indicates.
* Introduce `WP_Customize_Sidebar_Section` PHP class, and `SidebarSection` JS class.
* Move logic for determining whether a sidebar section is active from the `SidebarControl` to `SidebarSection`.

props westonruter.
fixes #30235.

git-svn-id: https://develop.svn.wordpress.org/trunk@30329 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-customize-control.php

@@ -1115,17 +1115,6 @@ class WP_Widget_Area_Customize_Control extends WP_Customize_Control {
 		<?php
 	}
 
-	/**
-	 * Whether the current sidebar is rendered on the page.
-	 *
-	 * @since 4.0.0
-	 * @access public
-	 *
-	 * @return bool Whether sidebar is rendered.
-	 */
-	public function active_callback() {
-		return $this->manager->widgets->is_sidebar_rendered( $this->sidebar_id );
-	}
 }
 
 /**