Improve plot size handling

[philippjfr]

• Improves size handling of Vega, Plotly and DeckGL
• Ensure custom models are cleaned up correctly

[codecov]

Codecov Report

Merging #4522 (f56a545) into main (64e8d7b) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #4522      +/-   ##
==========================================
+ Coverage   83.38%   83.40%   +0.02%     
==========================================
  Files         246      246              
  Lines       36270    36270              
==========================================
+ Hits        30242    30252      +10     
+ Misses       6028     6018      -10     

Flag	Coverage Δ
ui-tests	39.22% <100.00%> (+0.01%)	⬆️
unitexamples-tests	73.59% <0.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
panel/tests/ui/test_reactive.py	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

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