Preprocess: Automated summaries compatibility

[VesnaT]

Issue

The Preprocess widget will start crashing once biolab/orange3#5308 is merged.

Includes

• Code changes
• Tests
• Documentation

[janezd]

Auto summaries were broken (duplicated) because the widget hacks away the inherited signal by not deriving the class Inputs from base class' Inputs. Wouldn't this work (with also adding the declaration about signal renaming, for backward compatibility)?

     class Inputs(OWPreprocess.Inputs):
        data = Input("data", Corpus)

Or even, if muß es sein,

     class Inputs(OWPreprocess.Inputs):
        data = Input("corpus", Corpus)

With this, set_data will be able to call the inherited method~~, but auto summary will be called just once~~?

I just added a test; it is called twice, but sets the same thing. This is unwelcome, but shouldn't have any consequences. (I need to find a way to avoid this, though.)

[janezd]

I checked how signals are collected: without some overhaul there, I don't see a better solution than the one in this PR.