Splices a stream asynchronously.
When the operation is finished
callback will be called.
You can then call
gOutputStreamSpliceFinish to get the
result of the operation.
gOutputStreamSpliceAsync(object, source, flags = "G_OUTPUT_STREAM_SPLICE_NONE", io.priority = 0, cancellable = NULL, callback, user.data = NULL)
For the synchronous, blocking version of this function, see