[FIX] settings: Store settings version in the serialized defaults

[ales-erjavec]

Issue

Widget settings version were not stored in the serialized defaults, but were still passed to OWWidget.migrate_settings with an incorrect version of 0.

One example where this caused an error (due to wrong migration)

• Open canvas, add 'Test & Score', change the Sampling option to Test on test data, close canvas
• Open again, add 'Test & Score', observe that no Sampling option is selected at all. Without changing anything add a single file and learner input to the widget.

-------------------------- AssertionError Exception ---------------------------
Traceback (most recent call last):
  File "/Users/aleserjavec/workspace/orange3/Orange/canvas/scheme/widgetsscheme.py", line 839, in process_signals_for_widget
    widget.handleNewSignals()
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/evaluate/owtestlearners.py", line 486, in handleNewSignals
    self.__update()
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/evaluate/owtestlearners.py", line 826, in __update
    assert False, "self.resampling %s" % self.resampling
AssertionError: self.resampling 6

Description of changes

Store settings version in the serialized defaults.

Includes

• Code changes
• Tests
• Documentation

[jerneju]

https://sentry.io/biolab/orange3/issues/316851292/

[codecov-io]

Codecov Report

Merging #2631 into master will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2631      +/-   ##
==========================================
+ Coverage   75.01%   75.18%   +0.17%     
==========================================
  Files         331      331              
  Lines       58109    57796     -313     
==========================================
- Hits        43588    43456     -132     
+ Misses      14521    14340     -181