Customizer panels:

* Close all accordion sections when going in and out of panels.
* Fix panel positioning when top-level Customizer controls are scrolled down.
* Keyboard accessibility: only focus on visible elements and transfer between them as needed when navigating around panels.

props celloexpressions. see #27406.

Built from https://develop.svn.wordpress.org/trunk@29035


git-svn-id: http://core.svn.wordpress.org/trunk@28823 1a063a9b-81f0-0310-95a4-ce76da25c4cd