Add types for all Widget and Pane class variables

[philippjfr]

Lots and lots of types.

[codecov]

Codecov Report

Merging #3604 (c5583dc) into master (0749371) will decrease coverage by 0.21%.
The diff coverage is 83.87%.

@@            Coverage Diff             @@
##           master    #3604      +/-   ##
==========================================
- Coverage   82.65%   82.43%   -0.22%     
==========================================
  Files         203      203              
  Lines       27698    27824     +126     
==========================================
+ Hits        22894    22937      +43     
- Misses       4804     4887      +83     

Impacted Files	Coverage Δ
panel/io/notifications.py	63.63% <ø> (ø)
panel/widgets/texteditor.py	85.71% <50.00%> (-14.29%)	⬇️
panel/layout/grid.py	74.49% <57.14%> (-0.59%)	⬇️
panel/widgets/ace.py	92.10% <57.14%> (-7.90%)	⬇️
panel/pane/idom.py	28.16% <62.50%> (+0.96%)	⬆️
panel/pane/streamz.py	67.44% <62.50%> (-4.36%)	⬇️
panel/pane/echarts.py	75.00% <70.00%> (-3.34%)	⬇️
panel/pane/vega.py	78.80% <70.00%> (-1.20%)	⬇️
panel/pane/ipywidget.py	50.00% <72.72%> (ø)
panel/pane/perspective.py	59.31% <72.72%> (-0.53%)	⬇️
... and 35 more

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