Tabulator: allow to set selectable=1

[maximlt]

Addresses #3074

By just removing the default value of True of the select_mode model property as suggested in the issue which indeed allows to declare that only 1 row can be selected.

[codecov]

Codecov Report

Merging #3295 (b22e451) into master (1559e72) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3295   +/-   ##
=======================================
  Coverage   83.33%   83.33%           
=======================================
  Files         195      195           
  Lines       26374    26374           
=======================================
  Hits        21980    21980           
  Misses       4394     4394           

Impacted Files	Coverage Δ
panel/models/tabulator.py	94.44% <100.00%> (ø)

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