Now the accessibility for the single select box better matches what
is picked up for a native select.
This fixes an issue with us always setting `aria-activedescendant`,
which caused Orca to always say the full path to the selected
result, instead of saying that the box was a combobox with a
selection. This means that the `aria-activedescendant` attribute
will now only be set when the dropdown is open.
This also switches the results list from a listbox to a tree, which
is what Firefox tells screen readers that a standard select is.
Combined with a change to use `role=group` and `aria-label` for
option group labels, screen readers will now announce the group
that they are in as well.
cdfa0499f4