QSqlTableModelPrivate::ModifiedRow guard private data

QSqlTableModelPrivate::ModifiedRow guard private data

Even though ModifiedRow is not part of the public API, guarding its
data helps make clear the intended patterns of use.

"op" and "primaryValues" are read-only after construction.

setValue() encourages maintainers to let ModifiedRow manage the
"generated" flags of the record.

The primeInsert() signal still exposes the actual record, including
the "generated" flags, which is the justification for recRef().

Change-Id: I16d1610a8f9233af78b90662b08706b48ea19c41
Reviewed-by: Yunqiao Yin <charles.yin@nokia.com>