Make QDateTime::fromString()/Time::fromString() adhere to ISO 8601.

Make QDateTime::fromString()/Time::fromString() adhere to ISO 8601.

Currently QDateTime::fromString and QTime::fromString do not correctly
handle fractional minutes and, in some cases, fractional seconds.

In the case of reading fractional minutes, it has been decided to
ignore invalid characters outside of the 5 character portion that
we're interested in (see code comments in fromStringImpl() for
info on why we read 5 digits). The motive is that there is a
performance penalty for calling mid to get the portion of surplus
string and also for converting to it to a float. This is also in
line with what QDate does with surplus characters, for example.

Task-number: QTBUG-14418
Task-number: QTBUG-25387
Change-Id: Ib742fe80686aff3c3770b995678cf838fb4e3bb4
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>