Customizer: Fix previewing and applying widgets when previewing another theme.

* Unset `wp_get_sidebars_widgets()`' non-admin cache var `$_wp_sidebars_widgets` in Customize theme preview.
* Add `WP_Customize_Setting::$dirty` so that settings can be initially-dirty when the Customizer loads.
* Mark `old_sidebars_widgets_data` setting initially-dirty.
* Mark all `sidebars_widgets` settings as initially-dirty during theme switch.

props westonruter.
see #31484.

git-svn-id: https://develop.svn.wordpress.org/trunk@31705 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/customize.php

@@ -250,6 +250,7 @@ do_action( 'customize_controls_print_scripts' );
 		$settings['settings'][ $id ] = array(
 			'value'     => $setting->js_value(),
 			'transport' => $setting->transport,
+			'dirty'     => $setting->dirty,
 		);
 	}