Ensure GridSpec override handles duplicate matches

[philippjfr]

The GridSpec override logic matched overlaps for each cell which meant that the same item could be recorded for deletion multiple times which could fail. We now deduplicate the matches using a set operation and to be sure made the deletion robust to a KeyError.

Fixes #2106

[codecov]

Codecov Report

Merging #2150 (cf4cebe) into master (56ce69c) will decrease coverage by 0.04%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #2150      +/-   ##
==========================================
- Coverage   83.81%   83.77%   -0.05%     
==========================================
  Files         182      182              
  Lines       21672    21675       +3     
==========================================
- Hits        18165    18158       -7     
- Misses       3507     3517      +10     

Impacted Files	Coverage Δ
panel/layout/grid.py	75.07% <71.42%> (-0.38%)	⬇️
panel/tests/pane/test_base.py	88.23% <0.00%> (-11.77%)	⬇️
panel/io/reload.py	64.36% <0.00%> (-2.30%)	⬇️
panel/pane/base.py	87.89% <0.00%> (-0.90%)	⬇️

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