Implemented improved setitem semantics

[philippjfr]

No description provided.

[codecov-io]

Codecov Report

Merging #264 into master will increase coverage by 0.36%.
The diff coverage is 99.04%.

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
+ Coverage   88.81%   89.17%   +0.36%     
==========================================
  Files          34       34              
  Lines        5336     5523     +187     
==========================================
+ Hits         4739     4925     +186     
- Misses        597      598       +1

Impacted Files	Coverage Δ
panel/tests/test_layout.py	99.47% <100%> (+0.31%)	⬆️
panel/layout.py	95.2% <96.82%> (+0.44%)	⬆️

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