Add a DiscretePlayer class

[philippjfr]

Adds a DiscretePlayer class which is analogous to DiscreteSlider, i.e. rather than start and end values it has a options list, which it iterates over.

• Adds unit tests

[codecov-io]

Codecov Report

Merging #171 into master will increase coverage by 0.34%.
The diff coverage is 93.87%.

@@            Coverage Diff            @@
##           master    #171      +/-   ##
=========================================
+ Coverage   90.05%   90.4%   +0.34%     
=========================================
  Files          30      30              
  Lines        4698    4823     +125     
=========================================
+ Hits         4231    4360     +129     
+ Misses        467     463       -4

Impacted Files	Coverage Δ
panel/tests/test_widgets.py	100% <100%> (ø)	⬆️
panel/widgets.py	95.22% <90.9%> (-0.35%)	⬇️
panel/param.py	93.98% <0%> (+3.7%)	⬆️

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