Selector widgets: confusing re-selection

[ceball]

Changing values in python code does not update widgets in the notebook; that is a known limitation. However, after interacting with a widget, its value ought to reflect the underlying object's value

E.g. re-selecting the currently selected value in a dropdown where the underlying object is set to a different value is confusing. The widget gives a visual indication that something has happened, but nothing has happened.