Widget Customizer: Fix 'WidgetCustomizerPreview' is undefined error in IE8/9.

* Move `WidgetCustomizerPreview` to `wp.customize.WidgetCustomizerPreview`
* Move `WidgetCustomizerPreview_exports` to export_preview_data()
* Use `_wpWidgetCustomizerPreviewSettings` to transfer settings to `wp.customize.WidgetCustomizerPreview`

see #27485.
Built from https://develop.svn.wordpress.org/trunk@27653


git-svn-id: http://core.svn.wordpress.org/trunk@27496 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Widget Customizer: Use Underscores `indexOf` for IE8 compatibility.

props adamsilverstein.
see #27485.
Built from https://develop.svn.wordpress.org/trunk@27654


git-svn-id: http://core.svn.wordpress.org/trunk@27497 1a063a9b-81f0-0310-95a4-ce76da25c4cd