Check all possible date time fields before giving up and labeling as unknown

1. Make a list of all fields that store an actual date value by running multiple different photo/video files through exiftool.
2. Arrange this list in order of most likely to least likely (approximately) to contain an accurate date.
3. Make exifsort look through these fields in order, and only give up and label as unknown if none of the date fields contain a valid date.