Refresh content from [default] url using a selector for the content

[fragoulis]

When url is left blank the widget uses the current url as source.
On metabox refresh the widget would just replace the content with the whole response (which is the whole page).
Make it so that, given a selector, the metabox contents are updated only with the contents of the matching element living inside the response.

That would only work with html response.
It is kinda like the default CGridView behavior when it updates its dataset.