DropdownCascade does not have multiple values support

[mkrecek234]

DropdownCascade disables multiplesupport. This can be easily activated if we change the DropdownCascade's getJsValues function to:

/**
     * Will mark current value as selected from a list
     * of possible values.
     *
     * @param string|int $value the current field value
     */
    private function getJsValues(array $values, $value): array
    {
        foreach ($values as $k => $v) {
            if (!$this->multiple) {
                if ($v['value'] === $value) {
                    $values[$k]['selected'] = true;

                    break;

                }
            } else {
                if (in_array($v['value'], explode(',', $value))) {
                    $values[$k]['selected'] = true;
                }

            }
        }

        return $values;
    }

Plus we have to remove line

ui/src/Form/Control/DropdownCascade.php

Line 117 in 6b503a7

$this->multiple = false;

Happy to submit via PR, but unfortunately can only provide a demo example, but no behat test.

[mvorisek]

should be solved as part of #1020

[mvorisek]

PR welcomed.