Return if filename is in the editor stack.

        Args:
            filename: Name of the file to search for.  If filename is None,
                then checks if any file is open.

        Returns:
            True: If filename is None and a file is open.
            False: If filename is None and no files are open.
            None: If filename is not None and the file isn't found.
            integer: Index of file name in editor stack.

def is_file_opened(self, filename=None):
        """Return if filename is in the editor stack.

        Args:
            filename: Name of the file to search for.  If filename is None,
                then checks if any file is open.

        Returns:
            True: If filename is None and a file is open.
            False: If filename is None and no files are open.
            None: If filename is not None and the file isn't found.
            integer: Index of file name in editor stack.
        """
        if filename is None:
            # Is there any file opened?
            return len(self.data) > 0
        else:
            return self.has_filename(filename)