Draft: lua-factory: fix usage of grl_related_keys API
Depends: grilo!73
There were a few unclear distinctions as how to handle GrlRelatedKeys and GrlData as we could get GrlData value from GrlRelatedKeys.
As GrlReatedKeys can get more complex, it is necessary to make a distinction. If we want to fetch a metadata-key that is not binded to another metadata-key, we should use grl_data_* and grl_media_* APIs. Otherwise, we should fetch the GrlRelatedKeys and use their api to fetch values.
Related: grilo#145