Thumbnails update

Here is an attempt at solving two redmine bugs or feature requests: 

- https://med.inria.fr/redmine/issues/1778: flipped thumbnails
- https://med.inria.fr/redmine/issues/337: long thumbnail generation

The idea is that I switched to the same idea we used for vtk thumbnails. For feature 337, I would advocate going one step further, which is removing all thumbnails methods and adapting medDatabasePreview to handle this. But I feel like this branch is already a good start.

Tell me what you think