Allow to assign and retreive a thumbnail to a Genre

Currently the medialibrary doesn't provide thumbnails for genre. It's up to the application to generate its own. Though it would be nice if the medialibrary can cache the thumbnail and handle its lifecycle.

the API might be the same as the one in media

        virtual const std::string& thumbnailMrl( ThumbnailSizeType sizeType ) const = 0;
        virtual bool hasThumbnail( ThumbnailSizeType sizeType ) const = 0;
        virtual bool setThumbnail( const std::string& mrl, ThumbnailSizeType sizeType ) = 0;