Improve and standardize selection behavior of Tabulator

[philippjfr]

The old table had a more intuitive selection behavior which used Ctrl/Meta keys to toggle selection of multiple rows and the shift key for range selection. The Tabulator has a default behavior where clicking on rows simply toggles selection on and off. This changes the Tabulator selection behavior to be consistent with the old behavior but also allowing users to toggle the previous behavior by setting selectable='toggle'.

Fixes #2089

[codecov]

Codecov Report

Merging #2230 (06aece8) into master (6af7bb5) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2230      +/-   ##
==========================================
- Coverage   83.66%   83.66%   -0.01%     
==========================================
  Files         182      182              
  Lines       21866    21868       +2     
==========================================
+ Hits        18295    18296       +1     
- Misses       3571     3572       +1     

Impacted Files	Coverage Δ
panel/models/tabulator.py	100.00% <100.00%> (ø)
panel/widgets/tables.py	84.26% <100.00%> (+0.05%)	⬆️
panel/io/reload.py	64.36% <0.00%> (-2.30%)	⬇️
panel/pane/idom.py	27.40% <0.00%> (+0.20%)	⬆️

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