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This extends the existing behavior which only showed a message only when there were no widget areas rendered in the preview. The number of non-rendered widget areas is indicated. Also removes needles deletion of `wp.customize.Widgets.data.l10n` property which hindered plugins. See #33567, #33052. Fixes #39087. Built from https://develop.svn.wordpress.org/trunk@40312 git-svn-id: http://core.svn.wordpress.org/trunk@40219 1a063a9b-81f0-0310-95a4-ce76da25c4cd