[FIX] Select Rows: Fix incorrectly stored values in settings

[janezd]

Issue

Select Rows stored values of discrete variables as indices. If the data changed so that values were reordered, it chose wrong values. If values were removed, it could crash with index out of range.

To reproduce, construct File -> Select Rows -> Select Rows. Load Iris, obviously.

• In the second Select Rows, choose the second Iris type.
• In the first Select Rows remove the first type and remove unused classes.

After this the second Select Rows will have selected the (originaly) third (now second) Iris type.

Alternatively:

• In the second Select Rows select the last Iris type
• In the first Select Rows remove one type and remove unused classes.

This will crash the second Select Rows widget.

Description of changes

• Use DomainModel for combos.
• Internally store variables as instances of Variable; context handler converts this to the usual str, int presentation. With this, the context handler has an instance of variable, which it needs to see the values.
• Context handler stores values as strings. Conversion from and to int is done in context handler to minimize the changes to the widget's code.

Includes

• Code changes
• Tests

[codecov]

Codecov Report

Merging #4798 into master will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4798      +/-   ##
==========================================
+ Coverage   83.86%   83.95%   +0.08%     
==========================================
  Files         281      276       -5     
  Lines       56804    55990     -814     
==========================================
- Hits        47638    47005     -633     
+ Misses       9166     8985     -181