Add the Direct Download link to the page on a UrlFetched event

Calling _add_link() asynchronously will make it easy to add some URL
processing mechanism between _get_URL() and _add_link() in case the URL
we get needs to be processed somehow before being added to the page as
a Direct Download Link.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>