Detect nested symlinks through a separate directory tree

This became necessary because the file tree stores the images directly,
so directories could no longer be inserted into that tree. As a side
effect, --low-memory can no longer lead to a symlink-loop, only to
duplicate images.
Remove file_sorted_tree

Now that the nested-symlink problem is solved via a separate tree, the
only improvement brought by the sorted list is to help users who
unintentionally specified a parameter twice or who symlinked an image
multiple times. But both cases admit intentional use, and we prevented
users from that before. Therefore I think removing this feature is a
good idea.