Factor out NamedListLike baseclass

[philippjfr]

Makes the inheritance hierarchy a little nicer allowing other components to inherit NamedListLike behavior without explicitly subclassing the NamedListPanel class, which also includes a lot of functionality related to rendering child models.

[codecov]

Codecov Report

Merging #2127 (692b2be) into master (4904cfd) will decrease coverage by 0.02%.
The diff coverage is 90.32%.

@@            Coverage Diff             @@
##           master    #2127      +/-   ##
==========================================
- Coverage   84.50%   84.47%   -0.03%     
==========================================
  Files         182      182              
  Lines       21272    21301      +29     
==========================================
+ Hits        17975    17995      +20     
- Misses       3297     3306       +9     

Impacted Files	Coverage Δ
panel/layout/base.py	90.20% <90.32%> (-0.05%)	⬇️
panel/tests/pane/test_base.py	88.23% <0.00%> (-11.77%)	⬇️
panel/pane/base.py	87.89% <0.00%> (-0.90%)	⬇️

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