Support transform of GValue types
With some WIP around unserializing JSON but also related to existing API's, when we want to set a GValue to a GrlMedia with a metadata-key type that don't perfect match with GType of that GValue, we would get error.
To workaround the error, we would need to either ignore the pair metadata-key/value or to transform it. I believe that we can support g_value_transform() without drawbacks in the core itself, instead of doing it in Lua-Factory or in the unserialization code.
Added a few tests. Thanks for any feedback ;)