Fix to make disabled buttons behave correctly

As ticket [2532775](http://yuilibrary.com/projects/yui3/ticket/2532775) describes, disabled buttons are still clickable and only visually appear to be disabled.  This is because the call to `this._uiSetDisabled` routes to Widget's `_uiSetDisabled` instead of ButtonCore's `_uiSetDisabled`.  These commits fix that issue and add in relevant tests.

More investigation will be done on Button to determine if this is the most appropriate fix, or if some more work needs to be done on the architecture of the component.