macosx: Implement a movies library section

AFAICT the media library does not as of yet actually classify movies -- from its MetadataAnalyzer::parseVideoFile:

        }
        show->addEpisode( *media, seasonId, episodeId, std::move( episodeTitle ) );
    }
    else
    {
        // How do we know if it's a movie or a random video?
    }

Nevertheless, movies are a type exposed by this library and I assume we are interested in presenting them to the user. So this MR adds most of the infrastructure for adding a movies library section, even if it is not yet functional:

Signed-off-by: Claudio Cambra developer@claudiocambra.com