Make QTime::toString output milliseconds for Qt::TextDate, Qt::ISODate.

Section 4.2.2.4 of ISO 8601 allows for decimal fraction representations
of dates and times. Currently, when calling
QDateTime::toString(Qt::TextDate) or QDateTime::toString(Qt::ISODate),
the milliseconds will be omitted. However,
QDateTime::fromString(str, Qt::TextDate) and
QDateTime::fromString(str, Qt::ISODate) already support decimal
fraction representations, so this patch just adds this support to
QTime::toString, and hence QDateTime::toString().

Task-number: QTBUG-30250
Change-Id: If58e4b3d3105322c51d11a76b832e5e634d8991f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>