Widget Customizer: Restore highlighting of widgets in preview.

props westonruter.
see #27358.
Built from https://develop.svn.wordpress.org/trunk@27584


git-svn-id: http://core.svn.wordpress.org/trunk@27427 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Widget Customizer: Move preview script to wp_default_scripts().
Built from https://develop.svn.wordpress.org/trunk@27585


git-svn-id: http://core.svn.wordpress.org/trunk@27428 1a063a9b-81f0-0310-95a4-ce76da25c4cd