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
Make `<audio>` elements in playlists responsive.

See #27625.



git-svn-id: https://develop.svn.wordpress.org/trunk@27894 602fd350-edb4-49c9-b593-d223f7449a82
Call MediaElement's shim method for `setSrc()` when dynamically updating the playing source in playlists, which will properly set the source for mobile.

Make the labels for playlist items slightly smaller to account for size on mobile.

Fixes #27625.



git-svn-id: https://develop.svn.wordpress.org/trunk@27895 602fd350-edb4-49c9-b593-d223f7449a82
Merge branch 'master' into a11y