Update ButtonGroup value when changing options

[hoxbro]

Fixes #2998

[hoxbro]

I have an idea of how to fix the failing test but don't know when I have time to take a look at it. So you can update/close this PR if necessary @philippjfr.

[codecov]

Codecov Report

Merging #2999 (cc08dd9) into master (f5a099d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2999      +/-   ##
==========================================
+ Coverage   83.24%   83.26%   +0.01%     
==========================================
  Files         190      190              
  Lines       25014    25029      +15     
==========================================
+ Hits        20824    20840      +16     
+ Misses       4190     4189       -1     

Impacted Files	Coverage Δ
panel/tests/widgets/test_select.py	100.00% <100.00%> (ø)
panel/widgets/select.py	94.36% <100.00%> (+0.28%)	⬆️
panel/widgets/player.py	80.00% <0.00%> (-3.06%)	⬇️
panel/models/reactive_html.py	79.57% <0.00%> (-0.71%)	⬇️
panel/tests/test_reactive.py	98.86% <0.00%> (+0.01%)	⬆️
panel/template/base.py	78.76% <0.00%> (+0.04%)	⬆️
panel/reactive.py	76.23% <0.00%> (+0.48%)	⬆️

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[philippjfr]

Fabulous, thank you!