…roposal to resolve select2#3106 with some limitations for manual selection changes (as noted).

- This class has to keep both display and selection ordering for all options in order to manage selection order, while maintaining the default results/query ordering (when `sorter` is not used, etc).
- Notable but unfortunate limitation: Setting the control value using `jQuery.val([...])` with multiple items or manually toggling option `selected` (without a `change` event) still cannot preserve the value ordering.
  - The partial workaround for this is to manually set your selected options with the `data-selection-order` attribute before setting val() if possible.
  - It may also be possible to enhance the decorator to watch for manual changes via MutationObserver. TBD.

New options:
- `keepSelectionOrder`
- `trackManualSelectionOrder`

Update docs.
Add tests.

- Refactor initial DOM option and change event processing.
- Refactor option item/data to hold displayOrder/selectionOrder values, and inspect DOM attributes as way to manually set/update as needed (ie: when adding options, setting values, etc).
- Fix issues with using `ArrayData` and subclasses like `AjaxData`.

SelectAdapter fix:
- Update `SelectAdapter.prototype._normalizeItem` to properly set `id` from `text` for `<option>Text</option>` (no `value`) scenarios. This allows finding these options by value/id properly.

Update CHANGELOG.md with `SelectionOrder` decorator changes.