Rename and move RefreshableDataView

[pawamoy]

It's confusing because it is a view and we import it from views module but actually we use it as an Item in boxes! Besides it's uses the term "data" but truly it works only for charts (with display=Item.AS_HIGHCHARTS), when using it as an Item.

Of course if user defines a child of RefreshableDataView, he can return everything he wants from the get function, and use it as he wishes in a custom template.

I think RefreshableContent would be a good compromise. But then where to put it?

[pawamoy]

Renamed PartialResponse since it returns JSON as update content.