Fix support for copying cells and creating new views in JupyterLab

[philippjfr]

Fixes holoviz/pyviz_comms#3

See bokeh/bokeh#12195 for more discussion

[codecov]

Codecov Report

Merging #3652 (dc486b3) into master (33df87c) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3652      +/-   ##
==========================================
- Coverage   82.32%   82.29%   -0.03%     
==========================================
  Files         205      205              
  Lines       27893    27887       -6     
==========================================
- Hits        22963    22950      -13     
- Misses       4930     4937       +7     

Impacted Files	Coverage Δ
panel/io/callbacks.py	75.19% <0.00%> (-2.50%)	⬇️
panel/io/reload.py	69.56% <0.00%> (-2.18%)	⬇️
panel/tests/command/test_serve.py	83.54% <0.00%> (-1.46%)	⬇️
panel/util.py	85.60% <0.00%> (-0.38%)	⬇️
panel/io/server.py	76.35% <0.00%> (-0.21%)	⬇️
panel/io/state.py	68.05% <0.00%> (-0.20%)	⬇️
panel/pane/idom.py	28.36% <0.00%> (+0.19%)	⬆️

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