Adds `aria-selected` to child elements (WIP)

I still feel that the `core-selector` should be responsible for this....but I started adding semantics to the basic-selector (and basic-list-box). This communicates to assistive technology which child item is active.

I could use advice on which superclass method to override in basic-selector for updating the current child with `aria-selected="true"`. In core-selector, I easily accomplished this by putting logic in the `applySelection` method.