Fix Tabs.append and Plotly.applies bugs

[philippjfr]

Fixes #280

[codecov-io]

Codecov Report

Merging #282 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #282      +/-   ##
==========================================
+ Coverage   88.09%   88.11%   +0.01%     
==========================================
  Files          44       44              
  Lines        5638     5645       +7     
==========================================
+ Hits         4967     4974       +7     
  Misses        671      671

Impacted Files	Coverage Δ
panel/layout.py	95.2% <100%> (ø)	⬆️
panel/plotly.py	100% <100%> (ø)	⬆️
panel/tests/test_param.py	99.61% <100%> (ø)	⬆️
panel/tests/test_layout.py	99.48% <100%> (ø)	⬆️

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