Prevent to be invoked afterAdd when sorted an item.

AfterAdd callback has been invoked at "after add an item" and "after sort an item".
I don't hope that afterAdd be invoked when an item sorted.

```javascript
function onAfterAdd(element) {
   // If merged this request,
   // this function will be invoked only after added a new item
}
```

```html
<ul data-bind="sortable: { data: items, afterAdd: onAfterAdd }">
    <li data-bind="text: $data"></li>
</ul>
```