Helper function for constructing AsyncInitable object.
This is similar to @new but also initializes the object asynchronously.
When the initialization is finished,
callback will be called. You can then call new_async.end
to get the new object and check for any errors.
the I/O priority of the operation
optional Cancellable object, null to ignore.
the value of the first property, followed by other property value pairs, and ended by null.
a TaskReadyCallback to call when the initialization is finished
the name of the first property, or null if no properties
the data to pass to callback function