Hide widget area when closing a widget

After closing a widget, the widget area is empty, and should be hidden. Added the `hide` function in `WidgetView` next to `show`, and call on close of the model. 
I don't see how this could work.  Won't the following incorrectly cause the widget area to be hidden?

```python
w1 = widgets.Button()
w2 = widgets.Button()
display(w1, w2)
w1.close()
```