Improve page loads in the theme customizer by layering loading iframes....

Improve page loads in the theme customizer by layering loading iframes. Automate refreshing, but debounce multiple refresh events to prevent hammering the server with requests. see #19910.

git-svn-id: https://core.svn.wordpress.org/trunk@20031 1a063a9b-81f0-0310-95a4-ce76da25c4cd