[FIX] Scatter Plot: Fix a error when restoring from pre DomainModel workflows

[ales-erjavec]

... when the attr_* were stored by name.

For example open the second workflow (Interactive Visualizations) in the 'MenuBar->Help->Examples'

[codecov-io]

Current coverage is 89.38% (diff: 100%)

Merging #1672 into master will not change coverage

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[janezd]

I knew that #1608 breaks compatibility, but decided against introducing legacy patches.

I changed my mind. I don't oppose to adding this PR. Yet this example shows we need a general solution. 7e454bd is going to create the same problem in Sieve.

I suppose we need settings versions and a method OWWidget.settings_migration(old_settings) that returns migrated settings (or patches the given setting in-place). The method would be called before settings are applied to the widget. Classes like DomainContextHandler would provide utility functions such as those in this PR. @astaric?

[janezd]

Please wait with merging this; I will try to refactor it into a more general solution. Give me one day.

[astaric]

One day is gone. A general solution turned out to be more complicated. While it may come at a later time, I am merging this now as it fixes a known bug.

The general solution turned out more complicated and will be done at a later date.