Ensures global notification object can be used inside notebook callbacks

[philippjfr]

Fixes #4230

[codecov]

Codecov Report

Merging #4331 (7cc81f5) into main (47f231b) will decrease coverage by 44.76%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##             main    #4331       +/-   ##
===========================================
- Coverage   82.74%   37.99%   -44.76%     
===========================================
  Files         240      232        -8     
  Lines       34558    34358      -200     
===========================================
- Hits        28596    13054    -15542     
- Misses       5962    21304    +15342     

Flag	Coverage Δ
ui-tests	37.99% <0.00%> (-0.31%)	⬇️
unitexamples-tests	?

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Impacted Files	Coverage Δ
panel/io/state.py	35.93% <0.00%> (-34.71%)	⬇️
panel/__main__.py	0.00% <0.00%> (-100.00%)	⬇️
panel/models/katex.py	0.00% <0.00%> (-100.00%)	⬇️
panel/models/mathjax.py	0.00% <0.00%> (-100.00%)	⬇️
panel/models/perspective.py	0.00% <0.00%> (-100.00%)	⬇️
panel/pane/vtk/synchronizable_deserializer.py	0.00% <0.00%> (-97.59%)	⬇️
panel/models/ace.py	0.00% <0.00%> (-91.67%)	⬇️
panel/tests/widgets/test_input.py	8.86% <0.00%> (-91.14%)	⬇️
panel/tests/io/test_embed.py	8.58% <0.00%> (-90.86%)	⬇️
panel/models/terminal.py	0.00% <0.00%> (-90.63%)	⬇️
... and 168 more

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