add_status_code_handler
Description:
public uint add_status_code_handler (string @signal, uint status_code, Callback callback, void* user_data)
  
  Adds a signal handler to this for signal.
Similar to [func@GObject.signal_connect], but the callback will only be run if this has the 
      status status_code.
signal must be a signal that will be emitted after this's status is set (this means it can't be 
      a "wrote" signal).
Parameters:
| this | a Message | 
| status_code | status code to match against | 
| callback | the header handler | 
| signal | signal to connect the handler to. | 
| user_data | data to pass to  | 
Returns:
| the handler ID from [func@GObject.signal_connect] |