[ENH] Edit Domain: Change types of multiple variables

[janezd]

Issue

Fixes #6088.

Based on #6415 to avoid conflicts.

Description of changes

Allows changing the variable type for multiple variables at once.

As discussed with @markotoplak, editing other properties, even labels, would be confusing because controls would have to be disabled if selected variables differ in those properties. Also, the widget already has over 3000 lines of complex code.

The only feature we could easily add would be a tri-state checkbox for unlinking.

The widget has four separate editors for different variable types, which are stacked into a single editor. This PR implements a fifth editor, which is blank (except for the combo with type) and can handle multiple variables. Existing editors are kept as they are, that is, they accept a single variable. The "top editor" has separate methods for single and multiple variables. One might think that the four editors could accept lists instead of single variables to avoid having duplicated methods in the top editor. I thought so for long time, too, but the code was just getting more and more complex. This solution is simpler.

Includes

• Code changes
• Tests
• N/A Documentation

[codecov]

Codecov Report

Merging #6426 (6ee8479) into master (78a8d71) will increase coverage by 0.02%.
The diff coverage is 97.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6426      +/-   ##
==========================================
+ Coverage   87.63%   87.66%   +0.02%     
==========================================
  Files         321      321              
  Lines       69254    69374     +120     
==========================================
+ Hits        60693    60816     +123     
+ Misses       8561     8558       -3     

[markotoplak]

/rebase

[markotoplak]

@janezd, just to double check: if I understood this code correctly, it does not in any way change how settings are stored (because they are always saved per variable), and we thus do not need to increase the settings version.