I did not adjust the DateTimePicker element as the sent format is
controlled by the useragent due to either the datetime-local input
type or a javascript datetimepicker. (which does not exist yet, though)
Mentioning the RFC 3339 format such a picker expects is very likely
going to lead to confusion on an end user's side otherwise.
refs #7947
In case createElements() would still return the elements while requiring
the caller to add them to the form all form dependent configurations get
lost. (displaygroups, belongTo, ...) Wizards or parent forms can still
retrieve only input relevant fields by just calling createElements() and
getElements().
refs #5525
