Customizer: Improve parity between JS Setting models in preview with JS Setting models in pane.

* Ensure that Setting `Value` objects in preview get initial `_dirty` flag set if values among POST data.
* Upon `saved` event, send `saved` message to preview with the `response` to trigger `saved` event there.
* Reset `_dirty` flag for all setting `Value` objects in preview upon `saved`.
* Continue to create settings synced from pane even after initial bootstrap, and create them as dirty.
* Ensure that `id` property is set for setting `Value` objects in preview.

See #27355.
Fixes #35616.

Built from https://develop.svn.wordpress.org/trunk@36407


git-svn-id: http://core.svn.wordpress.org/trunk@36374 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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

@@ -805,6 +805,7 @@ final class WP_Customize_Manager {
 			'activePanels' => array(),
 			'activeSections' => array(),
 			'activeControls' => array(),
+			'_dirty' => array_keys( $this->unsanitized_post_values() ),
 		);
 
 		if ( 2 == $this->nonce_tick ) {