Add MenuButton widget

[philippjfr]

Wraps the Dropdown widget in bokeh, which generates a Menu which allows listening to the selected item.

• Closes Add Dropdown widget #1410
• Add tests
• Add docs

[codecov]

Codecov Report

Merging #1533 into master will decrease coverage by 0.02%.
The diff coverage is 69.04%.

@@            Coverage Diff             @@
##           master    #1533      +/-   ##
==========================================
- Coverage   85.73%   85.71%   -0.03%     
==========================================
  Files         145      145              
  Lines       16015    16034      +19     
==========================================
+ Hits        13731    13743      +12     
- Misses       2284     2291       +7     

Impacted Files	Coverage Δ
panel/widgets/button.py	79.41% <68.29%> (-6.31%)	⬇️
panel/widgets/__init__.py	100.00% <100.00%> (ø)

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