Allow for filtering the notifications

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### Feature description
The current notification system piles up quite some information that cannot be removed (#5585). An alternative to (permanently) removal of the notifications would be to allow to classify and filter them accordingly.

I can think of thinks like
- system messages
- calendar events
- birthdays
- app-specific events (separated by app)
- file changes/updates/shares/...
- core messages
- found updates for apps/core
- connection issues
- conflicting files (possible #4954)
- ...

It would maybe make the removal of notifications less critical if one could toggle a filter which events the user want to see and which should be hidden. That would allow also a sensible default per user (some might be tech-savvy while other want the bare minimum or distraction).