Support float point number formatting for point labels

class Q_CHARTS_EXPORT QPointLabelFormat {
public:
    QString stringFormat;
    char xFloatFormat;
    int xPrec;
    char yFloatFormat;
    int yPrec;

    QPointLabelFormat(QString stringFormat_, char xFloatFormat_ = 'g', int xPrec_ = 6, char yFloatFormat_ = 'g', int yPrec_ = 6);

    bool operator==(const QPointLabelFormat &format) const;
    bool operator!=(const QPointLabelFormat &format) const { return !(*this == format); }
};