Speed up dynamic layout updates

[philippjfr]

Addresses some of the slowness when dynamically updating a layout encountered in #4976

• Cache loading_css (loading this from file each time is expensive)
• Bokeh property access is expensive so we minimize it inside Panel._compute_sizing_mode
• Preprocessors currently always iterate over the entire object tree, we implement an alternative preprocessor signature that is given access to both the Panel object that was changed and all submodels that were unchanged.
• Do not add Link preprocessor unless Link is added
• Do not run Link preprocessor unless Document has links

Before

After

[codecov]

Codecov Report

Merging #4989 (4ced6a9) into main (0c2690c) will increase coverage by 10.00%.
The diff coverage is 78.94%.

@@             Coverage Diff             @@
##             main    #4989       +/-   ##
===========================================
+ Coverage   73.44%   83.45%   +10.00%     
===========================================
  Files         271      271               
  Lines       38293    38322       +29     
===========================================
+ Hits        28125    31981     +3856     
+ Misses      10168     6341     -3827     

Flag	Coverage Δ
ui-tests	40.82% <32.89%> (?)
unitexamples-tests	73.43% <77.63%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
panel/layout/grid.py	75.13% <23.07%> (-0.61%)	⬇️
panel/layout/accordion.py	86.55% <66.66%> (-0.74%)	⬇️
panel/layout/card.py	92.53% <66.66%> (+0.11%)	⬆️
panel/theme/base.py	87.67% <66.66%> (-0.79%)	⬇️
panel/io/resources.py	87.26% <83.33%> (+0.02%)	⬆️
panel/layout/base.py	91.82% <94.73%> (+0.08%)	⬆️
panel/io/convert.py	72.18% <100.00%> (+46.99%)	⬆️
panel/layout/tabs.py	90.83% <100.00%> (+0.07%)	⬆️
panel/links.py	83.37% <100.00%> (+0.43%)	⬆️
panel/reactive.py	81.11% <100.00%> (+1.00%)	⬆️
... and 2 more

... and 58 files with indirect coverage changes

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