[FIX] DataProjectionWidget: Update combos on new data

[VesnaT]

Issue

Fixes #4391

Description of changes

Includes

• Code changes
• Tests
• Documentation

[janezd]

At first I didn't understand why this fixes the problem. It is amazing that the widget worked with wrong variables in the model -- I guess it's only because we eliminated Variable.make, so the wrong variable matched the same-named variable from the new domain. :)

I added a test to convince myself that it's really about wrong variables in the model. And because it's prudent to have this test to prevent somebody from "re-optimizing" the code in the future.

[codecov]

Codecov Report

Merging #4405 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4405   +/-   ##
=======================================
  Coverage   87.43%   87.44%           
=======================================
  Files         405      405           
  Lines       74061    74073   +12     
=======================================
+ Hits        64758    64772   +14     
+ Misses       9303     9301    -2     

[VesnaT]

Thanks for the test. I tried to write one, but it kept passing with the old code.