Add support for default field value in MySQL driver

Default field values does not work correctly with Qt 4.x for MySQL databases. The feature was working in Qt 3.x

This means the code as follows does not work anymore with Qt4:

const QSqlRecord &tableInfo = GetDatabase().record(QString(m_strTableName));
QVariant xDefaultValue = tableInfo.field(uiIndex).defaultValue();

For MySQL databases the defaultValue() call in above code will always return an invalid variant, whereas the following call produces the expected result:

QString strName = tableInfo.field(uiIndex).name();

The defaultValue API does not work because QSqlField::setDefaultValue method is not called in the MySQL driver in qToField method.
For other database types such as SQLite or PSQL the QSqlField::setDefaultValue is called in corresponding places.