Fix change event triggered twice.

According to:
```js
if (this.hasNewRoot) {
    this.dragRootEl.trigger('change');
}
```
the change event is triggered on the new root on `dragStop`. However when you drag the item back inside its original root, this leads to trigger the change event twice on the original root.

This PR set `this.hasNewRoot` to `true` only if when the new root is different from the original root from which the drag start.