Refactor handling of Plotly updates

[philippjfr]

Refactors the Plotly pane a little bit and ensures that JS events trigger updates to plot.

Part of the problem here is that all property updates are handled via the same update path so to avoid running this multiple times the actual rendering is triggered by a change to the _render_count. However a user triggering updates manually, e.g. via JS will not know to increment the render count. Therefore we set a timeout that ensures that if _render_count hasn't been incremented within 250 ms of the an update to one of the properties then it is incremented automatically.

Fixes #2239

[codecov]

Codecov Report

Merging #2251 (92cc835) into master (74088e7) will decrease coverage by 0.00%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##           master    #2251      +/-   ##
==========================================
- Coverage   83.64%   83.64%   -0.01%     
==========================================
  Files         183      183              
  Lines       21894    21900       +6     
==========================================
+ Hits        18313    18318       +5     
- Misses       3581     3582       +1     

Impacted Files	Coverage Δ
panel/pane/plotly.py	91.42% <95.65%> (+0.89%)	⬆️
panel/io/reload.py	64.36% <0.00%> (-2.30%)	⬇️

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