[ENH] Heatmap: Allow labeling by any variable

[janezd]

Issue

Fixes #4206.

Description of changes

• Switch do using DomainModel to get everything for free.
• Store the annotation variable as an instance of Variable instead of integer indices into domain.
• Use the original domain and not just the numeric features for context!

Settings cannot be migrated because they were broken. The annotation variable was stored as an index into the original domain, while context was stored based on filtered domain. The two could mismatch and settings could have been stored incorrectly, so they can't be safely migrated.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4209 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4209      +/-   ##
==========================================
+ Coverage   85.95%   85.95%   +<.01%     
==========================================
  Files         394      394              
  Lines       70153    70145       -8     
==========================================
- Hits        60302    60296       -6     
+ Misses       9851     9849       -2