Serialization: Add grl_media_serialize_as and grl_media_unserialize_from
This MR was now rebased in top of !73 to have a clear definition about GrlData's metadata-keys and metadata-keys that belong to GrlRelatedKeys in GrlData.
This MR is preparing the code base to add JSON format too with !76
The goal is to have a proper API to increase the formats that we can transform from/to a GrlMedia.
Note that we already do something like that in lua-factory (tests)
This URI serializer still needs some improvements but that is not on my priority right now. I've left a few TODO in the test/serialize.c for future reference.
Edited by Victor Toso