Refactor OWVisWidget.
Fixed a bug introduced in 4b803479.
Do not try to match non context settings.
Refactor OWVisWidget.

- Replace callbacks with method attributes_changed, which is called
whenever shown attributes or hidden attributes change.
- shown_attributes and hidden_attributes are now private, as they
  contain tuples needed for list items. Attribute names can be accessed
  using properties with the same names. Setter for shown_attributes
  replaces setShownAttributeList method.