Use zim as a photo album

Integrate photo album features in zim. Allow using it as a travel journal and also add captions to photos.

Implies
- Albums by trip / date
- Album view with thumbnails
- Caption per image / link between image and journal text

Requires:
- special store and page types to handle the album
- framework to embed thumbnail view and image view in the interface
- rating images, to only show the best ones in the journal, hiding additional shots
- slide show mode with captions

Discussion:
- How to integrate the album view and the journal view, e.g. write story in journal, than link images to story and show excerpts in the caption when viewing a slideshow?
  - automatic linking by timestamps ???