[IMP] 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.