filterFunctionFactory (filterFunctionFactory) {
        if (!arguments.length) {
            return this._filterFunctionFactory;
        }
        this._filterFunctionFactory = filterFunctionFactory;
        return this;
    }
This function will be called with the search text, it needs to return a function that will be used to
filter the data. The default function checks presence of the search text.
@example
// This is the default
function (query) {
    query = _normalize(query);
    return function (d) {
        return _normalize(d).indexOf(query) !== -1;
    };
};
@param {function} [filterFunctionFactory]
@returns {TextFilterWidget|function}