ComboBox. Support parameterless constructor. Update scenario demo

[fergusonr]

No description provided.

[tig]

Nice work.

See: #447

I was just reviewing all event code throughout the project and noted this:

• public event EventHandler<ustring> Changed;
  • The implementation has some issues:
    1. The name is "Changed" but the event actually represents "the selection has been confirmed". Rename it to SelectionChanged?
    2. It's sending the current text; best practice is for an event to provide previous value. I'm not sure which is more appropriate for this API, but if you KNOW users of the event are going to want the 'new' text, then keep it as is. If you are not sure, then my suggestion is to defined a SelectionChangedEventArgs class and include both the old and new value in it.
    2. Calls to Invoke are spread around. Ideally there would be public virtual void OnSelectionChanged() method that calls SelectionChanged?.Invoke.

[tig]

LGTM!