Add subclassed Tabs model that respects sizing_mode

[philippjfr]

The Bokeh Tabs model does not use the sizing_mode/width|height_policy to determine the width/height of the tab headers which means that dynamic tabs sometimes collapse their tabs headers when dynamic tabs are enabled because the content of the tabs is not sufficiently wide.

Fixes #1231
Fixes #3041
Fixes #3064

[codecov]

Codecov Report

Merging #3301 (db6facf) into master (d7a6b66) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3301      +/-   ##
==========================================
- Coverage   83.44%   83.35%   -0.10%     
==========================================
  Files         195      196       +1     
  Lines       26451    26485      +34     
==========================================
+ Hits        22072    22076       +4     
- Misses       4379     4409      +30     

Impacted Files	Coverage Δ
panel/layout/tabs.py	88.28% <100.00%> (+0.10%)	⬆️
panel/models/__init__.py	100.00% <100.00%> (ø)
panel/models/tabs.py	100.00% <100.00%> (ø)
panel/widgets/input.py	94.18% <0.00%> (-2.16%)	⬇️
panel/pane/ipywidget.py	50.00% <0.00%> (-1.36%)	⬇️
panel/pane/plot.py	58.37% <0.00%> (-1.21%)	⬇️
panel/io/ipywidget.py	0.00% <0.00%> (ø)

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