Use theme customizer instead of thickbox theme preview

[koopersmith]

The now old-school thickbox theme previewer is still used on the theme install/update screens. We should use the new customizer instead and completely deprecate theme-preview.js.

[koopersmith]

The attached patch replaces all remaining instances of the thickbox theme previewer. It also separates the inclusion of the customize loader from the WP_Customize class. The loader is just a container that requires a JS file and a small amount of markup.

[nacin]

Looks good.

[dd32]

Just a fly-by: Make sure you test the Bulk update as well as singular update, the bulk update runs through a iframe, whereas the latter doesn't.

[koopersmith]

Replying to dd32:

Just a fly-by: Make sure you test the Bulk update as well as singular update, the bulk update runs through a iframe, whereas the latter doesn't.

Yep, I tested bulk update — works for me. :)

[koopersmith]

In [20419]:

Replace all instances of thickbox theme preview with the theme customizer. fixes #20404.

• Use theme customizer in theme install/update screens.
• Separate the customize loader from the customizer. Use wp_customize_loader() to include the loader script and markup.
• Deprecated: wp-admin/js/theme-preview.js is now no longer used by core.