Fix crash when a track isn't loaded on ArtistAlbumWidget.get_songs

If you switch very fast from the 'Albuns' view to the 'Artists'
view, it crashes. Apparently it happens when a track is not loaded
yet, so methods such as get_url() return NoneType which cannot be
converted to 'str' implicitly. It needs to be converted to 'str'
explicitly to be used on logging.debug.
Translate 'All Artists' label

http://bugzilla.gnome.org/show_bug.cgi?id=705505