Names of models are not saved as settings

[janezd]

1. Put logistic regression on the canvas.
2. Edit its name
3. Save the schema
4. Close canvas
5. Open Orange and load the schema

The logistic regression name is reverted back to Logistic regression.

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The base class for learners sets self.learner_name = self.name in __init__, where self.name is the widgets class attribute name, which is set later in the derived classes. So learner_name = Setting(name) or learner_name = Setting(OWWidget.name) won't work since the name is only defined later. Having learner_name = Setting("") would require some magic.

OWBaseLearner already has a meta class. This may be the point where the setting can be properly initialized since the name is already set then.

[astaric]

I opted for learner_name=Setting(None) to allow saving of empty names in workflows. See #2608 for details.