Asynchronously gets the requested information about the files in a directory.
The result is a FileEnumerator object that will give out FileInfo objects for all the files in the directory.
For more details, see enumerate_children which is the synchronous version of this call.
When the operation is finished, callback will be called. You can then call 
              enumerate_children_async.end to get the result of the operation.
| this | 
                 input File  | 
            
| attributes | 
                 an attribute query string  | 
            
| flags | 
                 a set of FileQueryInfoFlags  | 
            
| io_priority | 
                 the I/O priority of the request  | 
            
| cancellable | 
                 optional Cancellable object, null to ignore  | 
            
| callback | 
                 a AsyncReadyCallback to call when the request is satisfied  | 
            
| user_data | 
                 the data to pass to callback function  |