[FIX] ScoringSheetViewer computing target class values from base values

[ZanMervic]

Issue

Fixes #6996

Description of changes

• Refactors the ScoringSheetViewer widget so the displayed values are computed from base values which never change.
• Sorts the attributes by importance (absolute value of coefficients), so the most important attributes are displayed on top.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.36%. Comparing base (7be875f) to head (f79ef51).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7025   +/-   ##
=======================================
  Coverage   88.36%   88.36%           
=======================================
  Files         332      332           
  Lines       73345    73361   +16     
=======================================
+ Hits        64813    64829   +16     
  Misses       8532     8532