Unicode support for JFIF and EXIF comments:

- use UTF8 for JFIF comment
- use Unicode (UCS-2) to write JPEG UserComment,
support charset specification when reading the UserComment
- add convertCommentValue method to DMetaData

Using UTF8 for JFIF is simple and easy and should work.

The UCS-2 support needs testing (and a decision if we
always want to write Unicode, or a way to find out when we need to
and when we can as well write ASCII)

CCBUG: 120241 114211