Comparing the json representation should handle idiosyncracies of python->json datastructures

[jasongrout]

The output widget is suffering from a problem in how we check for what messages to send back to the server. In the default from_json at

ipywidgets/ipywidgets/widgets/widget.py

Line 650 in 4ffa003

return x

, we just return the object. However, since outputs are in a tuple, but the incoming json is a list, the comparison at

ipywidgets/ipywidgets/widgets/widget.py

Line 599 in 4ffa003

and to_json(value, self) == self._property_lock[key]):

always fails. The problem is that the roundtrip from python -> json -> python changes tuples into lists. Basically, this problem means that the output is cleared and reset multiple times, causing flicker.

See #1522

CC @maartenbreddels