new feature: source/destination links between categories. Will we keep this
feature? Code is complicated and very few people will understand how it
works...
modification: #images.storage_category_id replaced by
#image_category.is_storage
improvement: many code refactoring to improve readibility
improvement: virtual category creation code was moved to a dedicated
function in order to be called from admin/cat_list.php and
admin/cat_modify.php (create a new destination category)