DesktopNotifierSync: Add on_* properties for class-level events

[PhrozenByte]

No description provided.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.87%. Comparing base (5dab588) to head (314ca91).
⚠️ Report is 57 commits behind head on main.

Files with missing lines	Patch %	Lines
src/desktop_notifier/sync.py	66.66%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #207      +/-   ##
==========================================
+ Coverage   85.06%   85.87%   +0.80%     
==========================================
  Files          10       10              
  Lines         951      984      +33     
==========================================
+ Hits          809      845      +36     
+ Misses        142      139       -3     

Flag	Coverage Δ
pytest	85.87% <66.66%> (+0.80%)	⬆️

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[samschott]

Apologies for the delay in reviewing this! The change LGTM, just a minor request for change.

[PhrozenByte]

Done. Since you haven't had other comments about #208 I've updated this PR in anticipation of #208 as well. Meaning: You must merge #208 first, then re-run CI (mypy fails until #208 is merged, but should succeed then), and then merge this PR.

[samschott]

I've removed the on_cleared getters and setters again to be able to merge this PR first.