Ensure data models are reused ensuring that notification state persists

[philippjfr]

Previously data models would be re-created whenever the property changed which was pointless and meant that any state set on them may not persist.

Fixes #7504
Fixes #4964

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 8 lines in your changes missing coverage. Please review.

Project coverage is 81.82%. Comparing base (ad154df) to head (1ece104).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/reactive.py	68.42%	6 Missing ⚠️
panel/tests/ui/io/test_notifications.py	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7560      +/-   ##
==========================================
+ Coverage   71.90%   81.82%   +9.91%     
==========================================
  Files         338      339       +1     
  Lines       51441    51476      +35     
==========================================
+ Hits        36990    42119    +5129     
+ Misses      14451     9357    -5094     

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