Issue #2668220 by amateescu, Jacine, Grimreaper: Add ability to configure the selection settings for the 'items' reference field and improve the UX of the queue edit form

Issue #2650528: When deleting an entityqueue that uses the 'Multiple subqueues' handler, its subqueues are not deleted together.

Issue #2645064: When adding new entity queue, if Handler value is not selected (though it's a mandatory field), it throws error

Issue #2620520: Recoverable fatal error: Object of class Drupal\Core\Link could not be converted to string in Drupal\Core\Database\Statement->execute()