Widget: Optimize attachment of the _untrackClassesElement listener

jQuery UI 1.13.0 changed the logic attaching the `_untrackClassesElement`
listener in the `_classes` widget method; one of the side effects was calling
`this._on` for each node that needed the listener. That caused a severe
performance degradation for large comboboxes as each `_on` jQuery UI call
causes a jQuery `add` call that calls Sizzle's `uniqueSort` underneath.

Instead, collect the nodes that need the listener and then, outside of the loop,
create a jQuery object out of them and attach the listener once. That's still
slower than the jQuery 1.12 version but only slightly: 936 ms to 1.03s on a very
large list on a recent MacBook Pro, compared to ~30 seconds before this patch.

Fixes jquerygh-2014

ui/widget.js

@@ -499,6 +499,8 @@ $.Widget.prototype = {
 		}, options );
 
 		function bindRemoveEvent() {
+			var nodesToBind = [];
+
 			options.element.each( function( _, element ) {
 				var isTracked = $.map( that.classesElementLookup, function( elements ) {
 					return elements;
@@ -508,11 +510,13 @@ $.Widget.prototype = {
 					} );
 
 				if ( !isTracked ) {
-					that._on( $( element ), {
-						remove: "_untrackClassesElement"
-					} );
+					nodesToBind.push( element );
 				}
 			} );
+
+			that._on( $( nodesToBind ), {
+				remove: "_untrackClassesElement"
+			} );
 		}
 
 		function processClassString( classes, checkOption ) {