Widget Customizer: Spinner goes away before preview finishes refreshing after widget change

[westonruter]

Now that all widget changes cause full preview refreshes (to be addressed in #27355), and such widget changes can take awhile to see reflected in the preview, especially if on a slow shared host or if the site has poor performance in general. Originally

In r27652, the previewer-loading class started to get removed whenever widget form update Ajax call returned, whereas before this was deferred until when the preview sent a Customizer synced message. This part of the commit should be reverted.

Reported here: ​https://wordpress.org/support/topic/long-delays-to-update-widget-preview-with-customizer?replies=2

[westonruter]

Restore deferral of previewer-loading class removal until preview is synced. ​https://github.com/x-team/wordpress-develop/compare/trac-27635

[ocean90]

In 27913:

Widget Customizer: Restore deferral of previewer-loading class removal until preview is synced.

props westonruter.
fixes #27635.