Returns a list of the hidden columns for this tree.
        
        :return     [<str>, ..]

def hiddenColumns( self ):
        """
        Returns a list of the hidden columns for this tree.
        
        :return     [<str>, ..]
        """
        output  = []
        columns = self.columns()
        for c, column in enumerate(columns):
            if ( not self.isColumnHidden(c) ):
                continue
            output.append(column)
        return output