Changed the way the image list is handled.

1/ The images on the thumbnail list are no longer identified by
their CSS id, but by their position on the list of images (more
comfortable for inserting images, that is not implemented yet).

2/ A nice red rectangle is added to show which image is selected on
the thumbnail list.
Insertion of slides works.

The taken slides are inserted after the selected image on the
list. If there's no image, there're simply added.
The preview doesn't works at the moment with inserted slides.
OK, inserted slides are now well previewed.

Just an index changement in the viewer actualisation.