Utility method that retrieves all children and recursively searches for all media objects that satisfy the given search expression in 
            this container.
          
            
              | expression | the search expression or `null` for wildcard | 
            
              | offset | zero-based index of the first object to return | 
            
              | max_count | maximum number of objects to return | 
            
              | total_matches | sets it to the actual number of objects that satisfy the given search expression. If it is not possible to compute this value (in a 
                timely mannger), it is set to '0'. | 
            
              | cancellable | optional cancellable for this operation |