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.


git-svn-id: https://develop.svn.wordpress.org/trunk@35724 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/customize/class-wp-customize-nav-menu-item-setting.php

@@ -119,15 +119,6 @@ class WP_Customize_Nav_Menu_Item_Setting extends WP_Customize_Setting {
 	 */
 	public $original_nav_menu_term_id;
 
-	/**
-	 * Whether or not preview() was called.
-	 *
-	 * @since 4.3.0
-	 * @access protected
-	 * @var bool
-	 */
-	protected $is_previewed = false;
-
 	/**
 	 * Whether or not update() was called.
 	 *

src/wp-includes/customize/class-wp-customize-nav-menu-setting.php

@@ -88,15 +88,6 @@ class WP_Customize_Nav_Menu_Setting extends WP_Customize_Setting {
 	 */
 	public $previous_term_id;
 
-	/**
-	 * Whether or not preview() was called.
-	 *
-	 * @since 4.3.0
-	 * @access protected
-	 * @var bool
-	 */
-	protected $is_previewed = false;
-
 	/**
 	 * Whether or not update() was called.
 	 *