Append the values to the available options doesn't work for multiselect

Other options will be never added permanently to the list of options using a multiselect widget (it works nice for single select).

Have a look at following code fragment:

 if (
      $element['select']['#value'] == 'select_or_other' &&
      !empty($element['other']['#value']) &&
      !isset($element['#options'][$element['other']['#value']])
    ) { ...

The first condition isn't fulfilled using multiselect widgets (it contains an array with key and value 'select_or_other' instead of a string 'select_or_other'). Perhaps we can simply add a condition for multiselect checking for key:

 if (
      ($element['select']['#value'] == 'select_or_other' || isset($element['select']['#value']['select_or_other'])) &&
      !empty($element['other']['#value']) &&
      !isset($element['#options'][$element['other']['#value']])
    ) { ...

For me, it works, but I'm not sure if this change is useful?