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-admin/js/customize-controls.js

@@ -3345,6 +3345,8 @@
 							value._dirty = false;
 						} );
 
+						api.previewer.send( 'saved', response );
+
 						api.trigger( 'saved', response );
 					} );
 				};