Check unused snapshot references

If you recorded a snapshot and then removed element from the gallery, no error will be shown, so it's very easy to forget to remove them.
We should implement a way to scan filesystem and if reference images do not match with what we have in ItemStore, show an error

Start by locating ItemStore and the code that records or consumes snapshot reference images, then inspect how the gallery stores those references on the filesystem. Done means scanning the filesystem, detecting references absent from ItemStore after an element is removed, and showing an error for each mismatch.

Written by the indexing model from the issue text.