Handle exif date setting a little better

- also check Exif.Image.DateTimeOriginal when reading the file
- set Exif.Image.DateTime to the current date as per Exif specs
- set *.DateTimeOriginal and *.DateTimeDigitized to the time from db so
  that time offsets assigned in the geotagging module is accounted for

The last part might not be exactly what people that scan and archive
analog images expect/want. However, they are not our primary audience,
and since they sport a more complicated workflow anyway it shouldn't be
too hard to reset these fields from the original source image after
export again. Possibly using Lua for simple automation.