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

fixes #27622.

git-svn-id: https://develop.svn.wordpress.org/trunk@27892 602fd350-edb4-49c9-b593-d223f7449a82
Cleanup [27892].

git-svn-id: https://develop.svn.wordpress.org/trunk@27893 602fd350-edb4-49c9-b593-d223f7449a82