Widget Customizer: Use postMessage to highlight widgets in preview or sections/controls in Customizer.

fixes #27622.
Built from https://develop.svn.wordpress.org/trunk@27892


git-svn-id: http://core.svn.wordpress.org/trunk@27723 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Cleanup [27892].
Built from https://develop.svn.wordpress.org/trunk@27893


git-svn-id: http://core.svn.wordpress.org/trunk@27724 1a063a9b-81f0-0310-95a4-ce76da25c4cd