Remove unused imports.

Signed-off-by: Aron Cedercrantz <aron@cedercrantz.se>
Rename songs() to getSongsRelation().

Signed-off-by: Aron Cedercrantz <aron@cedercrantz.se>
Add method to mark all songs of a party as played or not.

Signed-off-by: Aron Cedercrantz <aron@cedercrantz.se>