interactive.widget_from_abbrev should be a classmethod

[jdemeyer]

All the widget_from_... methods can be called from the interactive class (without an instance) because they are static methods. The only exception is widget_from_abbrev (which calls the other methods). Fix this by making widget_from_abbrev a class method. Note that it cannot be a static method, since we need to call other methods.

[SylvainCorlay]

Thanks, this makes perfect sense.