also mark images as removed/processed when clicking on them in thumbnail mode and --action is specifed

Note that the i == r check was never reached in the original code but may be
in the new variant. Since a shuffled list may well have one or two elements
retain their absolute position once in a while, this should not be a problem.

tests with 2- and 3-element lists show an even distribution amongst all
elements.

Not all functions were examined yet. Also TODO:
* check strdup / malloc calls (some are apparently unchecked in giblib),
  maybe use estrdup / emalloc for those
* Check includes and update dependency list if needed

They have been deprecated since 2011 and may be removed entirely in future
versions.