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.
This makes quite a few changes, one of the major ones being the
removal of classes for marking options as selected or selectable,
and instead using the ARIA attributes which should already be
present.
Placeholder support has been implemented as a separate module, so
any selection container should be able to be decorated and get
instant placeholder support. It hooks into the updating method of
selections, and determines when to display the placeholder based
on the options that are being updated.
It works in the same way as the old placeholders. If no options
are selected and being displayed, like in the case of a multiple
select, then the placeholder will always be shown. If one option
is being displayed, and the id of the placeholder matches the id
of the selected element, then the placeholder will be shown. This
is similar to the functionality that was present in Select2 2.x,
where the placeholder could be passed in as an object that would
be compared to the selection.
This still requires that, for single selects, the first element
must match the placeholder id. Because the default placeholder id
is a blank string, this will maintain backwards compatibility with
past versions where the first option should be blank. This can
still be overridden to point at a different id, keeping support
for systems where the placeholder doesn't use a blank value.
**Note:** This does not hide the blank option for single selects,
but that will still be maintained for backwards compatibility
within the results module. It will not depend on a placeholder
being present, but instead will hide any options with blank text.
