Read content from the given input stream and deserialize it, 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_deserialize_finish() to get the result of the operation.
GInputStream to read the serialized content from
the mime type to deserialize from
the GType to deserialize from
the I/O priority of the operation
callback to call when the operation is done