[ADD] URL widget for list view columns

I see a use case for specifying a column of a list/tree view as `widget="url"` for some cases where that column contains a hyperlink to an external web page.  This simple change modifies the content of a column (if there is content), so that the data is wrapped in a standard HTML anchor tag.

If the user does not specify a protocol (HTTP/S, etc.) the `//` (inherit current protocol) prefix is appended.