Add extrafanart and extrathumbs to library.

Adds ListItem.Art(extrafanartn) and ListItem.Art(extrathumbsn) where n=number of downloads.
This will break current implementation used in skins where they show the images in the extrafanart path, but has advantage that skins can check on existence of extrafanart and wont have to use dirty hacks to show extrafanart/extrathumbs.