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