A number of fixes for panel cleanup

[philippjfr]

No description provided.

[codecov-io]

Codecov Report

Merging #150 into master will decrease coverage by 0.02%.
The diff coverage is 86.95%.

@@           Coverage Diff            @@
##           master   #150      +/-   ##
========================================
- Coverage   90.02%    90%   -0.03%     
========================================
  Files          30     30              
  Lines        4473   4474       +1     
========================================
  Hits         4027   4027              
- Misses        446    447       +1

Impacted Files	Coverage Δ
panel/layout.py	95.13% <100%> (-0.07%)	⬇️
panel/holoviews.py	82.92% <100%> (+0.21%)	⬆️
panel/pane.py	82.12% <50%> (ø)	⬆️
panel/param.py	90.7% <75%> (-0.27%)	⬇️
panel/pipeline.py	84.82% <83.33%> (ø)	⬆️

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