Serialize content and write it to the given output stream, asynchronously.
The default I/O priority is
G_PRIORITY_DEFAULT (i.e. 0), and lower numbers
indicate a higher priority.
When the operation is finished, @callback will be called. You must then
content_serialize_finish() to get the result of the operation.
GOutputStream to write the serialized content to
the mime type to serialize to
the content to serialize
the I/O priority of the operation
callback to call when the operation is done