Add and clean up utilities for event handling

[philippjfr]

Utilities like hold and immediate_dispatch are incredibly valuable to optimize dispatching of events. I will follow up with a how-to on how to leverage them to get better control over when and how events are dispatched to avoid unnecessary re-rendering.

[codecov]

Codecov Report

Merging #5444 (f303749) into main (905a44b) will decrease coverage by 13.26%.
The diff coverage is 60.52%.

@@             Coverage Diff             @@
##             main    #5444       +/-   ##
===========================================
- Coverage   83.29%   70.04%   -13.26%     
===========================================
  Files         274      274               
  Lines       40057    40075       +18     
===========================================
- Hits        33366    28069     -5297     
- Misses       6691    12006     +5315     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	70.04% <60.52%> (-2.55%)	⬇️

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Files Changed	Coverage Δ
panel/io/document.py	46.26% <20.00%> (-5.61%)	⬇️
panel/io/model.py	62.18% <86.36%> (-0.43%)	⬇️
panel/io/__init__.py	75.00% <100.00%> (-20.00%)	⬇️

... and 86 files with indirect coverage changes

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