[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 88.62%. Comparing base (7be875f) to head (67f6d2d).
Report is 24 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7025      +/-   ##
==========================================
+ Coverage   88.36%   88.62%   +0.25%     
==========================================
  Files         332      332              
  Lines       73345    73381      +36     
==========================================
+ Hits        64813    65034     +221     
+ Misses       8532     8347     -185     

[janezd]

I simplified the initialization of coefficients from data -- some lines probably remained from the way it was implemented before. Can someone (@ZanMervic or @VesnaT) check it? Thanks!