DesktopNotifierSync: Add on_* properties for class-level events

[PhrozenByte]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 12 lines in your changes missing coverage. Please review.

Project coverage is 84.93%. Comparing base (5dab588) to head (94dfb9b).
Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
src/desktop_notifier/sync.py	66.66%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #207      +/-   ##
==========================================
- Coverage   85.06%   84.93%   -0.13%     
==========================================
  Files          10       10              
  Lines         951      976      +25     
==========================================
+ Hits          809      829      +20     
- Misses        142      147       +5     

Flag	Coverage Δ
pytest	84.93% <66.66%> (-0.13%)	⬇️

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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.