Add ability to make only certain Tabulator rows selectable

[philippjfr]

• Adds selectable='checkbox-single' option to disable (de)select-all in the header
• Add selectable_rows parameter which accepts a function which should return the indexes of the selectable rows given a DataFrame.

[codecov]

Codecov Report

Merging #2433 (e496acd) into master (87f4a44) will increase coverage by 0.01%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master    #2433      +/-   ##
==========================================
+ Coverage   83.44%   83.46%   +0.01%     
==========================================
  Files         183      183              
  Lines       22639    22679      +40     
==========================================
+ Hits        18892    18928      +36     
- Misses       3747     3751       +4     

Impacted Files	Coverage Δ
panel/widgets/tables.py	83.56% <87.50%> (-0.22%)	⬇️
panel/models/tabulator.py	100.00% <100.00%> (ø)
panel/reactive.py	75.00% <100.00%> (+0.03%)	⬆️
panel/tests/widgets/test_tables.py	99.29% <100.00%> (+0.03%)	⬆️

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