Added a base class for the selection container
Working on accessibility

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.
Add more keyboard support

When the main container is focused, you can now move the highlighted
result by using the up and down arrows on the keyboard.

This does not yet keep the highlighted result visible at all times,
and it only is implemented on single selects, but that will come in
the future.
Single select accessibility

Now the Select2 instance is correctly recognized as a combobox, and
the selected option is read aloud when it is focused, just like in
a standard select box.

This works by generating semi-random ids that are used for the
ARIA attributes.  These are not intended to be consistent by any
means, they are just generated to make the instance accessible by
screen readers.
Fixed keyboard scrolling

Now when scrolling through results with the keyboard, the
highlighted option will always be visible.
Changed tests to not use deepEqual

We should only be checking the option values that matter, such as
`id` and `text`, instead of checking all of the option values. This
will prevent unexpected breaking when new properties are added to
the options.

Existing properties should be covered by tests to avoid regressions.
Generate more consistent ids

A global id is generated for the container, and each further id
builds off of the global id.
Better accessibility for results

Options are correctly called out as focus moves around the results
list.
Accessibility matches better

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.