Added collectionView:canDragItemAtIndexes:forEvent:.

Reviewed by me.

AppKit/CPCollectionView.j

@@ -597,6 +597,10 @@
     if (![_selectionIndexes count])
         return;
 
+    if ([_delegate respondsToSelector:@selector(collectionView:canDragItemsAtIndexes:withEvent:)] &&
+        ![_delegate collectionView:self canDragItemsAtIndexes:_selectionIndexes withEvent:anEvent])
+        return;
+
     // Set up the pasteboard
     var dragTypes = [_delegate collectionView:self dragTypesForItemsAtIndexes:_selectionIndexes];