Sets the mapper for the given column name to the callable.  The inputed
        callable should accept a single argument for a record from the tree and
        return the text that should be displayed in the column.
        
        :param      columnName | <str>
                    callable   | <function> || <method> || <lambda>

def setColumnMapper(self, columnName, callable):
        """
        Sets the mapper for the given column name to the callable.  The inputed
        callable should accept a single argument for a record from the tree and
        return the text that should be displayed in the column.
        
        :param      columnName | <str>
                    callable   | <function> || <method> || <lambda>
        """
        columnName = nativestring(columnName)
        if ( callable is None and columnName in self._columnMappers ):
            self._columnMappers.pop(columnName)
            return
        
        self._columnMappers[nativestring(columnName)] = callable