Rank: Use VariableListModel to show decoration and tooltips

[janezd]

Issue

Fixes #6140.

Description of changes

Derives a new model from BarRatioTableModel, which delegates the first column to VariableListModel, which then provides the usual data (tooltips, decoration...) seen in other widgets.

It assumes that data is changed only through wrap, as is the actual case in the widget. If this is unacceptable, we could instead change VariableListModel's methods that generate tooltips into class methods or static methods, and call the from RankTableModel instead of having an actual instance of VariableListModel.

Includes

• [X] Code changes
• [X] Tests

[codecov[bot]]

Codecov Report

Merging #6142 (2592dc4) into master (fbb50d7) will increase coverage by 0.00%. The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6142   +/-   ##
=======================================
  Coverage   86.46%   86.46%           
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  Files         315      315           
  Lines       67645    67660   +15     
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+ Hits        58488    58501   +13     
- Misses       9157     9159    +2