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.


git-svn-id: https://develop.svn.wordpress.org/trunk@41869 602fd350-edb4-49c9-b593-d223f7449a82

src/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,