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 | 
| A list of media objects. |