also fixes the fact that the display format of QDateTimeEdit only accepts string formats, so in case of Qt::ISODate use a default date/time with zmne zone
this adds a new format of date/time to use Qt::ISODate format
the issue is that QDate::fromString does not recognize the t letter for the timezone in format
this a hack since any field_format needs to be tested if it corresponds to QgsDateTimeFieldFormatter::DEFAULT_ISO_FORMAT and in such case replace by Qt::ISODate instead of a format string
Run clang-tidy modernize-use-override to remove all the redundant
virtual keywords from overridden methods, and add some missing
overrides.
Another benefit is that this has also added the overrides
on destructors, which will cause a build failure if a base
class is missing a virtual destructor.
