Improve/introduce Customizer JavaScript models for Controls, Sections, and Panels.

* Introduce models for panels and sections.
* Introduce API to expand and focus a control, section or panel.
* Allow deep-linking to panels, sections, and controls inside of the Customizer.
* Clean up `accordion.js`, removing all Customizer-specific logic.
* Add initial unit tests for `wp.customize.Class` in `customize-base.js`.

https://make.wordpress.org/core/2014/10/27/toward-a-complete-javascript-api-for-the-customizer/ provides an overview of how to use the JavaScript API.

props westonruter, celloexpressions, ryankienstra.
see #28032, #28579, #28580, #28650, #28709, #29758.
fixes #29529.



git-svn-id: https://develop.svn.wordpress.org/trunk@30102 602fd350-edb4-49c9-b593-d223f7449a82