[FIX] owconfusionmatix: Add migrate_settings

[ales-erjavec]

Issue

ref #1794

For some period of time the 'selected_learner' property was changed from List[int] -> int (4e49bb3) and then back again (8a492d7)

When opening a workflow that was saved between that time (between releases 3.3.4 and 3.3.7) the widget would raise an exception

Traceback (most recent call last):
  File "/Users/aleserjavec/workspace/orange3/Orange/canvas/scheme/widgetsscheme.py", line 822, in process_signals_for_widget
    handler(*args)
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/evaluate/owconfusionmatrix.py", line 225, in set_results
    prev_sel_learner = self.selected_learner.copy()
AttributeError: 'int' object has no attribute 'copy'

Description of changes

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Current coverage is 88.96% (diff: 100%)

Merging #1796 into master will not change coverage

@@             master      #1796   diff @@
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  Files            82         82          
  Lines          8963       8963          
  Methods           0          0          
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  Hits           7974       7974          
  Misses          989        989          
  Partials          0          0          

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