Expose the orientation parameter of the button group widgets

[maximlt]

This PR simply exposes a Bokeh property not exposed by Panel.

Is the CSS of some templates to adapt for this kind of change or is this good to go?

[codecov]

Codecov Report

Merging #3083 (e23584b) into master (e66dea3) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3083   +/-   ##
=======================================
  Coverage   83.42%   83.42%           
=======================================
  Files         190      190           
  Lines       25134    25135    +1     
=======================================
+ Hits        20969    20970    +1     
  Misses       4165     4165           

Impacted Files	Coverage Δ
panel/widgets/select.py	94.40% <100.00%> (+0.03%)	⬆️
panel/widgets/tables.py	85.53% <0.00%> (-0.02%)	⬇️

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[philippjfr]

Should be good to go. Thanks @maximlt !