code: func ProgressBarNew() (*ProgressBar, error) {
  	c := C.gtk_progress_bar_new()
  	if c == nil {
  		return nil, nilPtrErr
  	}
  	return wrapProgressBar(glib.Take(unsafe.Pointer(c))), nil
  }
docs: // ProgressBarNew() is a wrapper around gtk_progress_bar_new().