This patch adds an argtype for GValue so that gda functions taking or returning a GValue* are wrapped by the code generator.

There is still a problem with functions returning a GdaDataModel: The datamodel implementation depends on the provider. When calling gda_data_model_execute_select_command() on a connection with a postgresql provider, the actual type of the returned GdaDataModel is GdaPostgresRecordset. However, this is private API and users should work with the GdaDataModel interface. When a GdaDataModel is returned, pygda wants to wrap the actual type, but it fails, because it does not know anything about GdaPostgresRecordset.

I am not sure how to fix that. A possible solution would be to define all the recordsets in gda.defs by hand and put the gda_postgres_recordset_get_type prototype and GDA_TYPE_POSTGRES_RECORDSET macro in gdaglue.h (for each provider libgda supports). The patch does not yet cover this because this approach seems like a bit of a hack to me. Perhaps someone comes up with a better idea.