[FIX] Fix slow clear/delete in 'Heat Map' 'Hier. Clustering', 'Distance Map'

[ales-erjavec]

Issue

owheatmap, owdistancemap and owhierarchicalclustering use QGraphicsLinearLayout to position text labels in a list. Clearing the QGraphicsLinearLayout however is slow (seems to be another quadratic time complexity).

Extract a more limited (but sufficient) TextListWidget from owsilhouetteplot and replace the use of QGraphicsLinearLayout.

Fixes gh-2663
Fixes gh-4349

Description of changes

• extract and extend the TextListWidget from owsilhouetteplot
• use the TextListWidget in owheatmap, owhierarchiclustering, owdistancemap

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4365 into master will increase coverage by 0.05%.
The diff coverage is 98%.

@@            Coverage Diff             @@
##           master    #4365      +/-   ##
==========================================
+ Coverage   87.13%   87.18%   +0.05%     
==========================================
  Files         399      401       +2     
  Lines       72901    72931      +30     
==========================================
+ Hits        63521    63585      +64     
+ Misses       9380     9346      -34

[thocevar]

Heat map visualizations with a lot of rows (e.g., Adult dataset) are now besides being faster to delete also faster to construct.

Distance map contains a quadratic number of elements anyway, therefore the difference is not that obvious.

Hierarchical clustering must be doing something strange besides this. It should containt a linear number of elements but is still slow to delete. I don't think it solves #2663.