Automatically set input/output summary

[janezd]

@irgolic said that we could get rid of duplicated code for setting and testing summaries (see #5002) by putting the code for setting the summaries and for calling them into Orange.widgets.widget.OWWidget. Discussion (@irgolic, @markotoplak, @janezd) led to the following.

1. Summary is set automatically under the following conditions:
   • widget has the default signal
   • the default signal is of type Table
   • widget does not set the flag, named sth like auto_set_summary (but better) to False (default is True).
2. For the sake of backward compatilibility or perhaps also for future convenience, calling set_input_summary/set_output_summary sets this flag to False.
3. The redundant code is removed from widgets, together with corresponding tests (see Assert minus, not empty string for empty brief msg #5002).

Afterthoughts:

If we do this, we can probably close biolab/orange-widget-base#101?

Output is easy. The obvious place to trigger this for output is Orange.widgets.widget.OWWidget.send.

For input, there is no such spot. orangewidget.utils.signals.InputSignal.__call__ is a decorator that registers input, but returns the method essentially undecorated. We could derive a new class, Orange.widgets.widget.Input that would wrap the method into one that would call set_input_summary. However, this solution not only introduces magic, but may also be inconsistent with what happens in the widget: the input signal handler may reject the data (e.g. sth. like self.data = None), but the summary would still be set.

[janezd]

Done in #5308.