refactored VCardTool::parseDateTime and VCardTool::createDateTime

- split createDateTime into createDate and createTime so they can be used separately to
  either export the date or the time. (createDateTime always exported the time because
  dateTime.time().isValid() returns true when dateTime has a valid date.)
- parseDateTime now also recognizes offsets from UTC (e.g. +0100 or -12:30)
- createDateTime can now write offsets from UTC
- when parseDateTime can't parse the time *timeValid is set to false (I would like to
  use this in my next patch to prevent that the birthday field is parsed with a time when
  no time is present)
- adjusted customidentifiertest and importexportvcardtest because they failed now
- added some tests for createDateTime and parseDateTime (I had to make them public therefore)

REVIEW: 128655