Adds correct semantics to basic-list-box

To be accessible, the basic-list-box and its child elements needs correct roles, states and properties. The technique in this PR uses `aria-activedescendant` to communicate which child item is selected. Further semantics have been added to `basic-selector`, PR here: https://github.com/basic-web-components/basic-selector/pull/1
The == is intentional. I wasn't sure if it were possible for getAttribute() to return `undefined`, so used == to pick up a check for that case too.