Do not freeze document models unless needed

[philippjfr]

Currently we freeze the Document model references every time a property is changed, triggering reference invalidation which is expensive. Instead we should check if we are adding/removing a model and only freeze the model references if that's the case.

[codecov]

Codecov Report

Merging #5864 (8036929) into main (ba97f63) will increase coverage by 1.59%.
The diff coverage is 80.76%.

@@            Coverage Diff             @@
##             main    #5864      +/-   ##
==========================================
+ Coverage   82.37%   83.96%   +1.59%     
==========================================
  Files         290      290              
  Lines       42296    42318      +22     
==========================================
+ Hits        34842    35533     +691     
+ Misses       7454     6785     -669     

Flag	Coverage Δ
ui-tests	40.89% <23.07%> (+2.49%)	⬆️
unitexamples-tests	72.34% <80.76%> (+<0.01%)	⬆️

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Files	Coverage Δ
panel/layout/base.py	90.66% <100.00%> (+0.03%)	⬆️
panel/io/document.py	71.68% <88.88%> (+25.42%)	⬆️
panel/layout/grid.py	74.40% <25.00%> (-0.13%)	⬇️

... and 16 files with indirect coverage changes

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