Ensure discrete slider respects layout opts

[philippjfr]

Fixes #1134

[codecov]

Codecov Report

Merging #1144 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1144      +/-   ##
==========================================
+ Coverage   87.82%   87.82%   +<.01%     
==========================================
  Files         105      105              
  Lines       12576    12577       +1     
==========================================
+ Hits        11045    11046       +1     
  Misses       1531     1531

Impacted Files	Coverage Δ
panel/widgets/slider.py	90.99% <100%> (+0.04%)	⬆️

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