Aggregate Columns: Add additional options for selection

[janezd]

Issue

Fixes #5643.

Description of changes

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #6056 (6282b98) into master (0524aa9) will increase coverage by 0.01%.
The diff coverage is 99.13%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6056      +/-   ##
==========================================
+ Coverage   86.65%   86.66%   +0.01%     
==========================================
  Files         315      315              
  Lines       67623    67715      +92     
==========================================
+ Hits        58597    58688      +91     
- Misses       9026     9027       +1     

[markotoplak]

Seems fine. There is some seemingly inconsistent behaviour with the Features input:

1. with 2 numerical features, the widget goes into the features input mode and disables the rest.
2. with 1 numerical and 1 categorical feature, the widget goes into the features input mode, disables the rest, and shows a warning,
3. with 2 categorical features, the widget DOES NOT go into the features input mode, but still shows a warning.

With (3) I'd expect that the widget goes into the features input mode (because of the input), and an error was shown. Or perhaps easier to program, the same warning + an error because no features can be aggregated.