Provide Viewer baseclass

[philippjfr]

Provides a Viewer baseclass which can be used to make custom components viewable (i.e. provide .servable, .show and _repr_mimebundle_ methods. A user just has to implement __panel__.

Closes #1979
Fixes #1981
Fixes #1980

[codecov]

Codecov Report

Merging #2045 (448b0cc) into master (20617c8) will decrease coverage by 0.36%.
The diff coverage is 64.70%.

@@            Coverage Diff             @@
##           master    #2045      +/-   ##
==========================================
- Coverage   84.30%   83.93%   -0.37%     
==========================================
  Files         178      178              
  Lines       20612    20203     -409     
==========================================
- Hits        17377    16958     -419     
- Misses       3235     3245      +10     

Impacted Files	Coverage Δ
panel/pane/base.py	90.38% <50.00%> (-1.56%)	⬇️
panel/viewable.py	71.64% <63.63%> (-0.35%)	⬇️
panel/pipeline.py	88.63% <75.00%> (-0.09%)	⬇️
panel/tests/pane/test_base.py	87.09% <0.00%> (-12.91%)	⬇️
panel/config.py	46.15% <0.00%> (-5.79%)	⬇️
panel/auth.py	36.51% <0.00%> (-3.37%)	⬇️
panel/pane/ipywidget.py	47.05% <0.00%> (-2.95%)	⬇️
panel/io/reload.py	63.95% <0.00%> (-2.72%)	⬇️
panel/io/callbacks.py	54.16% <0.00%> (-2.42%)	⬇️
panel/tests/template/test_manual.py	60.63% <0.00%> (-2.37%)	⬇️
... and 84 more

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[MarcSkovMadsen]

This is great