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.

git-svn-id: https://develop.svn.wordpress.org/trunk@27653 602fd350-edb4-49c9-b593-d223f7449a82