BUG: qtAttributeView reporting incorrect validity

The original logic used the visibility state of the association widget to determine if it's validity should factor into the qtAttributeView that owned the widget. Though this does work when the association widget is being initially constructed, it fails when the widget is being updated though its operation handler. My guess is that in this case the widget is currently hidden and hence its visibility method was returning false.

The fix was to store a member variable to indicate if the association widget is currently in use.