source to the location specified by destination
asynchronously. For details of the behaviour, see gFileCopy.gFileCopyAsync(object, destination, flags = "G_FILE_COPY_NONE", io.priority = 0, cancellable = NULL, progress.callback, progress.callback.data, callback, user.data = NULL)progress.callback is not NULL, then that function that will be called
just like in gFileCopy, however the callback will run in the main loop,
not in the thread that is doing the I/O operation.
When the operation is finished, callback will be called. You can then call
gFileCopyFinish to get the result of the operation.