Don't use names=traitlets.All in observe

[ianhi]

Problem

We use observe inherited from traitlets.HasTraits which uses names=traitlets.All as a default argument. This can have confusing effects for anyone who wants to manually observe a widget. For example see #3192

My sense is that for most of the core widgets people really only want to observe the value trait so it's a bit unnecessary to always have to do slider.observe(callback, names='value') or to add an if statement to the callback.

Proposed Solution

Override observe for the widget classes to give a default argument for names that is curated for the widget being used. So for example for an IntSlider we would change the default to be value.

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Marking as 8.0 due to back incompat.

[jasongrout]

In our dev meeting today, we discussed this and think that probably it would be too big of a change? Do you have a nice elegant solution we could look at?

[vidartf]

Closing for now.