digikam from trunk : improving image properties restoration in database using image metadata:

- Fix DMetadata method to get image Exif/Iptc tags properlly. (With the old implementation, Comments from Exif and Iptc, Rating from Iptc and are never checked duing a wrong validity test - stupid bug)
- Now at startup, these informations are backported to database :

==> Comments from JFIF section, or Exif UserComments tag, or Iptc Caption tag.
==> Date & time stamp from Exif dateTime tag or Iptc date & time tags.
==> Rating from Iptc Urgency tag !!! If you add new files in your Album library, witch are rated using Mapivi for example, digiKam items rating will be appear in main interface.

Nota : these updates in database are only performed to new files :

==> when all albums are parsed during statup (or manually from Tool menu).
==> when new items are downloaded using camera interface.
==> when a folder is imported from main interface.

TODO :

- Performed a database update at startup when files are already in database and when metadata have been changed outside digiKam (using ExifTools for example)
- Do something with digiKam Tags, since they are stored in IPTC Keywords tags. This is most complicated to do because there is no hierarchy between IPTC Keywords like with digiKam tags. We store only Tags name in IPTC keywords. I propose :

==> to check if a digiKam tags name already exist in database and taging automaticly item using it.
==> do nothing if Tags name do not exist (no new digiKam Tags will be created in database).
==> If dupplicate Tags name exist in digiKam database (for ex. Travel/City and Travel/France/City), use only the first Tag name found in database.

digiKam Tags <==> IPTC Keywords rules is a complex problem. Please give me your viewpoints into B.K.O. Thanks in advance
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