digikam from trunk : new fast image preview mode embedded on main interface.

- Separate Preview and Edit of picture. Preview is embedded into main interface, Edit is dedicaced to image editor.
- digiKam album content area can be now toogled between the current image collection thumbs or the current preview of image selected in collection
- To toogle between these views press F3. You can also press ESC to go out of preview mode.
- You can change current image preview using wheel mouse or these keyboard shorcuts : page up, page down, home, end.
- The preview image background use the current color of the selected digiKam theme.
- Preview of RAW files are computed by dcraw using -e option. It's very fast.

There is a screenshot at this url :

http://digikam3rdparty.free.fr/Screenshots/image_preview_on_main_interface.png


TODO :

- Extract JPEG, PNG, and TIFF preview from Exif Makernote using Exiv2. Need to adapt Exiv2 library for that.
- Loading preview image outside the main interface thread using a new digiKam KIO-Slave.
- Moving keyboard shortcuts of preview widget into main digiKam interface instance to share it with image properties sidebar tabs
- Add RMB menu in this mode or just a link to Edit current image in editor. Need users feedback for that.


BUG: 127991, 128669, 128914