update ember-popup-menu to fix broken tests
use stringify CP for aria-selected
don't instrument selection; just set the html

TODO: find a way to detect mutations to the active descendant and
update the label appropriately. Instrumentation seems like the
incorrect approach here, so I removed it
make nearestChild aware of whether a child is appended
use nearestChild to get the label and list on the menu
make the select menu list just a configuration list