Ensure that closed tabs update Tabs.objects

[philippjfr]

Fixes #621

[codecov]

Codecov Report

Merging #973 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #973      +/-   ##
==========================================
+ Coverage   85.69%   85.74%   +0.05%     
==========================================
  Files          98       98              
  Lines       11030    11072      +42     
==========================================
+ Hits         9452     9494      +42     
  Misses       1578     1578

Impacted Files	Coverage Δ
panel/tests/pane/test_holoviews.py	99.46% <ø> (-0.01%)	⬇️
panel/tests/test_layout.py	100% <100%> (ø)	⬆️
panel/tests/test_server.py	100% <100%> (ø)	⬆️
panel/layout.py	83.76% <100%> (+0.63%)	⬆️
panel/tests/test_reactive.py	100% <100%> (ø)	⬆️
panel/viewable.py	81.52% <100%> (+0.21%)	⬆️
panel/io/notebook.py	62.13% <100%> (+0.22%)	⬆️

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