Customizer: Use "(Untitled)" as site title if blogname is empty.

Fixes a layout issue in the Customizer UI. Also de-duplicate title display logic, outputting "Loading..." as site title in PHP with actual title being set upon Customizer `ready`. Also update the site title in response to a `blogname` setting change as opposed to `input` DOM events on the control.

Fixes #35579.


git-svn-id: https://develop.svn.wordpress.org/trunk@36388 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/script-loader.php

@@ -443,6 +443,7 @@ function wp_default_scripts( &$scripts ) {
 		'loginIframeTitle'   => __( 'Session expired' ),
 		'collapseSidebar'    => __( 'Collapse Sidebar' ),
 		'expandSidebar'      => __( 'Expand Sidebar' ),
+		'untitledBlogName'   => __( '(Untitled)' ),
 		// Used for overriding the file types allowed in plupload.
 		'allowedFiles'       => __( 'Allowed Files' ),
 	) );