Menu customizer: More clearly separate search results from available items.

Available items now fade from view while you're searching, and there is an explicit way to clear search results. No results gives a better message, though still brief this time around.

props valendesigns, designsimply, DH-Shredder, helen.
fixes #32710.


git-svn-id: https://develop.svn.wordpress.org/trunk@33511 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/ajax/CustomizeMenus.php

@@ -520,7 +520,7 @@ class Tests_Ajax_CustomizeMenus extends WP_Ajax_UnitTestCase {
 				array(
 					'success' => false,
 					'data'    => array(
-						'message' => 'No menu items found.',
+						'message' => 'No results found.',
 					),
 				),
 			),