[ENH] Predictions: Allow selecting a subset of rows

[janezd]

Closes #4869.

Includes

• Code changes
• Tests

[codecov]

Codecov Report

Merging #4871 into master will increase coverage by 0.16%.
The diff coverage is 99.32%.

@@            Coverage Diff             @@
##           master    #4871      +/-   ##
==========================================
+ Coverage   84.12%   84.28%   +0.16%     
==========================================
  Files         283      283              
  Lines       57819    58020     +201     
==========================================
+ Hits        48640    48904     +264     
+ Misses       9179     9116      -63     

[PrimozGodec]

If I first connect data and then model to the Predictions widget it fails when I select a row with:

Traceback (most recent call last):
  File "/Users/primoz/Documents/orange3/Orange/widgets/evaluate/owpredictions.py", line 1119, in select
    self.store.select(selection, flags)
  File "/Users/primoz/Documents/orange3/Orange/widgets/evaluate/owpredictions.py", line 1054, in select
    self.select_rows(rows, flags)
  File "/Users/primoz/Documents/orange3/Orange/widgets/evaluate/owpredictions.py", line 1074, in select_rows
    self._rows ^= rows
  File "/Users/primoz/miniconda3/envs/orange3/lib/python3.8/contextlib.py", line 120, in __exit__
    next(self.gen)
  File "/Users/primoz/Documents/orange3/Orange/widgets/evaluate/owpredictions.py", line 1104, in _emit_changed
    model.emit_selection_rows_changed(selected, deselected)
  File "/Users/primoz/Documents/orange3/Orange/widgets/evaluate/owpredictions.py", line 1151, in emit_selection_rows_changed
    self.selection_from_rows(selected),
  File "/Users/primoz/Documents/orange3/Orange/widgets/evaluate/owpredictions.py", line 1133, in selection_from_rows
    index = model.index
AttributeError: 'NoneType' object has no attribute 'index'

If first the model is connected and the data it works.

The workflow (I used Iris data)