Customize: Harden assignment of Customizer settings transports for selective refreshable widgets

Theme support for `customize-selective-refresh-widgets` can be added _after_ the logic for registering the settings for incoming widgets that have been changed. This is due to themes adding the theme support in `after_setup_theme` which is also the action where `WP_Customize_Widgets::register_settings()` is called. If these both happen at priority 10, which one is called first depends on which one was added first. The other issue is that at the time that `WP_Customize_Widgets::register_settings()` is called at `after_setup_theme`, it is called before `widgets_init` and thus no widgets are yet registered. This means that any settings registered at this point will always have a `refresh` transport even if the theme supports `customize-selective-refresh-widgets`, since the `WP_Widget` instance is not visible yet to see if it supports selective refresh.

The fix: Defer `WP_Customize_Widgets::register_settings()` from `after_setup_theme` to `widgets_init` at priority 95 when the widget objects have all been registered. Also, ensure that the preview filter for `sidebars_widgets` is added before the sidebars are iterated for adding the controls.

Props westonruter.
Fixes #36389.

git-svn-id: https://develop.svn.wordpress.org/trunk@37166 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -206,6 +206,19 @@ class WP_Customize_Setting {
 		}
 	}
 
+	/**
+	 * Reset `$aggregated_multidimensionals` static variable.
+	 *
+	 * This is intended only for use by unit tests.
+	 *
+	 * @since 4.5.0
+	 * @access public
+	 * @ignore
+	 */
+	static public function reset_aggregated_multidimensionals() {
+		self::$aggregated_multidimensionals = array();
+	}
+
 	/**
 	 * The ID for the current site when the preview() method was called.
 	 *