Customizer: Use visibility to hide available widgets and menu items panels from screen readers and to move them out of tab order.

props afercia.
fixes #32883.
Built from https://develop.svn.wordpress.org/trunk@33136


git-svn-id: http://core.svn.wordpress.org/trunk@33107 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/customize-widgets-rtl.css

@@ -350,10 +350,11 @@ body.adding-widget .add-new-widget:before {
 
 #available-widgets {
 	position: absolute;
-	overflow: auto;
 	top: 0;
 	bottom: 0;
 	right: -301px;
+	visibility: hidden;
+	overflow: auto;
 	width: 300px;
 	margin: 0;
 	z-index: 1;
@@ -435,6 +436,7 @@ body.adding-widget .add-new-widget:before {
 
 body.adding-widget #available-widgets {
 	right: 0;
+	visibility: visible;
 }
 
 body.adding-widget .wp-full-overlay-main {