Sets of gets element value
@param {String} val New element value. If not passed, current
value is returned
@returns {String}
function(val) {
			if (!_.isUndefined(val) && this._value !== (val = String(val))) {
				this.parent._updateSource(val, this.valueRange());
				this._value = val;
			}
			
			return this._value;
		}