Customize: Ensure that a setting (especially a multidimensional one) can still be previewed when the post value to preview is set after preview() is invoked.

* Introduce `customize_post_value_set_{$setting_id}` and `customize_post_value_set` actions which are done when `WP_Customize_Manager::set_post_value()` is called.
* Clear the `preview_applied` flag for aggregated multidimensional settings when a post value is set. This ensures the new value is used instead of a previously-cached previewed value.
* Move `$is_preview` property from subclasses to `WP_Customize_Setting` parent class.
* Deferred preview: Ensure that when `preview()` short-circuits due to not being applicable that it will be called again later when the post value is set.
* Populate post value for updated-widget with the (unsanitized) JS-value in `WP_Customize_Widgets::call_widget_update()` so that value will be properly sanitized when accessed in `WP_Customize_Manager::post_value()`.

Includes unit tests with assertions to check the reported issues and validate the fixes.

Fixes defect introduced in [35007].
See #32103.
Fixes #34738.

Built from https://develop.svn.wordpress.org/trunk@35724


git-svn-id: http://core.svn.wordpress.org/trunk@35688 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-customize-widgets.php

@@ -1380,7 +1380,7 @@ final class WP_Customize_Widgets {
 		 * in place from WP_Customize_Setting::preview() will use this value
 		 * instead of the default widget instance value (an empty array).
 		 */
-		$this->manager->set_post_value( $setting_id, $instance );
+		$this->manager->set_post_value( $setting_id, $this->sanitize_widget_js_instance( $instance ) );
 
 		// Obtain the widget control with the updated instance in place.
 		ob_start();