In the past, Select2 triggered the `select2-clearing` event when
clearing the placeholder. This has been switched to the `unselect`
event which is now triggered whenever the placeholder is cleared.
The placeholder can also be prevented if the `unselecting` event
is prevented.
This will also trigger the event when deleting every option from
the multiple select, so it is possible for the `unselecting` event
to be triggered multiple times when clearing a multiple select.
This adds tests to ensure that the `allowClear` option always works.
This closes https://github.com/select2/select2/issues/2954.
