Broken symlinks not reflected in Eclipse resources model

While working on https://github.com/eclipse-platform/eclipse.platform/pull/2793 I've stumbled once again over the fact, that broken symbolic links aren't shown by Eclipse at all.
```
# Create a link to not existing file
ln -s not_existing_file broken
ll broken
lrwxrwxrwx 1 someuser users 5 3. Jul 13:18 broken -> not_existing_file
```
So from Eclipse one can't see broken files or directory symbolic links & can't remove them.
Sometimes this would be really helpful to see there **are** broken links in projects, because some files or directories were moved without updating symlinks. In order to warn user about such links, one has to write custom code which would trigger external processes to detect broken FS state. Ideally Eclipse would also include broken symlinks into resources model, so one doesn't need to spawn external processes to detect them.

I have two patches for resources & platform UI, let explore if they are enough or not (I guess few tests would fail).

Below screenshots shows directory which is "empty" in current Eclipse.

Esamina le risorse esistenti e i platform UI patches menzionati nell’issue, quindi traccia il modo in cui le voci del filesystem vengono individuate e rappresentate nel modello delle risorse di Eclipse. Verifica il comportamento dell’UI interessata per la visualizzazione e la rimozione dei link simbolici non validi, e aggiungi o aggiorna i test per i link non validi a file e directory. Il lavoro è completo quando questi link compaiono nel modello e possono essere rimossi da Eclipse.

Scritto dal modello di indicizzazione a partire dal testo della issue.