Add CleanupJob for removing non-existent files

[koppor]

(This is #7288 (comment))

As user, I have renamed files, but JabRef still has the old links. JabRef perfectly links the new files, but also keeps the broken links. I want to have them removed.

Implement org.jabref.logic.cleanup.RemoveLinksToNotExistentFiles.

That class is similar to org.jabref.logic.cleanup.MoveFilesCleanup, but uses org.jabref.logic.externalfiles.LinkedFileHandler#findExistingFile. Links with org.jabref.model.entry.LinkedFile#isOnlineLink() do not exist in the file system, but have to kept, too.

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When testing, please make use of JUnit's @TempDir or let inspire yourself with org.jabref.logic.search.DatabaseSearcherWithBibFilesTest.

[WenJJ2000]

Hi, we (@WenJJ2000 @karlsb @burcukilic @muchembledMartin @LinusWallin) are a group of university students who would like to work on this issue if possible!

[github-actions]

As a general advice for newcomers: check out Contributing for a start. Also, guidelines for setting up a local workspace is worth having a look at.

Feel free to ask here at GitHub, if you have any issue related questions. If you have questions about how to setup your workspace use JabRef's Gitter chat. Try to open a (draft) pull-request early on, so that people can see you are working on the issue and so that they can see the direction the pull request is heading towards. This way, you will likely receive valuable feedback.

[karlsb]

Hi! We have implemented a cleanup job for removing non-existing files but do not know when to trigger it. Should there be a new field in the "cleanup entry" under "quality" in the app menu? Or should it be triggered automatically somewhere else? Is it in the scope of this issue?

[koppor]

1. Open Chocolate.bib
2. Select all entries
3. 
4. "Enable field formatters"

Add the cleanup to the checklist above. Place it Above "Convert to biblatex format".

If you want, you can improve the dialog to add some grouping to all file-related points (from "Upgrade external PDF/PS links" to your new entry"