Extract artist in ArtistAlbumWidget instead of passing it as a constructor
parameter

Fixes: #703388
Merge pull request #135 from gnome-prototypes-team/fixes5

Extract artist in ArtistAlbumWidget instead of passing it as a constructor  parameter
increase offset in _add_item if and only if the check on the if item
condition passed
Merge pull request #137 from gnome-prototypes-team/fixes5

increase offset in _add_item if and only if the check on the if item condition passed