Implement support for passing parameters by reference

[philippjfr]

Supersedes #4042

[codecov]

Codecov Report

Merging #4495 (ebf54f9) into main (ecd08bd) will increase coverage by 0.07%.
The diff coverage is 96.05%.

@@            Coverage Diff             @@
##             main    #4495      +/-   ##
==========================================
+ Coverage   82.21%   82.28%   +0.07%     
==========================================
  Files         243      243              
  Lines       35355    35485     +130     
==========================================
+ Hits        29066    29198     +132     
+ Misses       6289     6287       -2     

Flag	Coverage Δ
ui-tests	35.86% <48.68%> (+0.07%)	⬆️
unitexamples-tests	74.20% <96.05%> (+0.07%)	⬆️

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Impacted Files	Coverage Δ
panel/tests/widgets/test_base.py	98.36% <75.00%> (-1.64%)	⬇️
panel/pane/base.py	90.00% <93.75%> (+0.26%)	⬆️
panel/reactive.py	80.33% <95.89%> (+1.04%)	⬆️
panel/pane/ipywidget.py	55.00% <100.00%> (+3.21%)	⬆️
panel/param.py	86.54% <100.00%> (-0.13%)	⬇️
panel/pipeline.py	88.80% <100.00%> (+0.08%)	⬆️
panel/tests/pane/test_base.py	96.82% <100.00%> (+0.46%)	⬆️
panel/tests/test_reactive.py	99.15% <100.00%> (+0.06%)	⬆️
panel/util/__init__.py	87.60% <100.00%> (+0.05%)	⬆️
panel/widgets/ace.py	92.10% <100.00%> (ø)
... and 10 more

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[philippjfr]

I'm going ahead and merging this to be able to evaluate it as I re-write various examples and applications. How-tos around reactivity and parameter binding are already lacking so I'm going to add as many of those in one go as I can.