This is a followup to MDL-18301. That fix missed the following points:
1. On the edit categories and items screens, all items had an eye-con to
control the visibility, even if the visibility was controlled by the
module.
2. Changing the visibility of a grade category change the visibility of
all items within it, even if the visibility was controlled by the
module.
3. The quiz ingored $cm->visible when controlling whether its grade item
was visible.
