KMeans: Scale silhouette bars

[janezd]

Issue

Fixes #7217.

Description of changes

I noticed that even the Silhouette widget scales the bars.

In its current form, KMeans' silhouette bars are almost always of the same length, even for reasonably good clustering. Thus I just scale them, always. No checkboxes.

Includes

• Code changes
• Tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.03%. Comparing base (246474b) to head (e5a76d5).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7230   +/-   ##
=======================================
  Coverage   89.03%   89.03%           
=======================================
  Files         335      335           
  Lines       74243    74246    +3     
=======================================
+ Hits        66099    66107    +8     
+ Misses       8144     8139    -5     

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