Customize: Add `'save-request-params'` event on `wp.customize` for plugins to inject/modify params in `save` requests.

Allows feature plugins, for example, to supply a `customize_changeset_title` param instead of having to wrap `wp.customize.previewer.query`.

Props dlh.
Fixes #42206.

Built from https://develop.svn.wordpress.org/trunk@41869


git-svn-id: http://core.svn.wordpress.org/trunk@41703 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/js/customize-controls.js

@@ -589,6 +589,9 @@
 			data.customize_changeset_autosave = 'true';
 		}
 
+		// Allow plugins to modify the params included with the save request.
+		api.trigger( 'save-request-params', data );
+
 		request = wp.ajax.post( 'customize_save', data );
 
 		request.done( function requestChangesetUpdateDone( data ) {
@@ -7180,6 +7183,9 @@
 						query.customize_changeset_title = args.title;
 					}
 
+					// Allow plugins to modify the params included with the save request.
+					api.trigger( 'save-request-params', query );
+
 					/*
 					 * Note that the dirty customized values will have already been set in the
 					 * changeset and so technically query.customized could be deleted. However,

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-beta2-41868';
+$wp_version = '4.9-beta2-41869';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.