Viewer now updates when using depends on __panel__

[hoxbro]

Fixes #3136

I hope this was what was meant with using ParamMethod.

[codecov]

Codecov Report

Merging #3159 (1325cc4) into master (5bd9fc8) will decrease coverage by 0.03%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master    #3159      +/-   ##
==========================================
- Coverage   82.99%   82.96%   -0.04%     
==========================================
  Files         192      192              
  Lines       25482    25504      +22     
==========================================
+ Hits        21150    21160      +10     
- Misses       4332     4344      +12     

Impacted Files	Coverage Δ
panel/viewable.py	69.36% <33.33%> (-1.09%)	⬇️
panel/pane/markup.py	96.17% <0.00%> (-1.05%)	⬇️
panel/widgets/input.py	96.29% <0.00%> (-0.46%)	⬇️
panel/widgets/tables.py	84.76% <0.00%> (-0.22%)	⬇️
panel/io/save.py	62.60% <0.00%> (+0.32%)	⬆️

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