Less digits from QVariant::toString() for float and double values

Users have requested to have similar amount of digits from QVariant::toString() as in Qt4 .

toString() is used often to display values on GUI and it is unusable if the output has too many digits.

Example:

float value = 252.7;
 QVariant var = QVariant::fromValue(value);
 qDebug() << var.toString(); //Qt4: "252.7", Qt5: "252.6999969482422"

Users wish to have similar output from QVariant::toString() as in Qt4.