Fix the missing support for save_callback being a function

[qzminski]

Right now the callbacks inside save_callback must be provided as arrays:

'save_callback' => [
    ['class1', 'method1'],
    ['class2', 'method2'],
],

However, Contao also supports functions. This pull request fixes this, so functions can be provided as well:

'save_callback' => [
    ['class1', 'method1'],
    static function($value) {
        return $value;
    }
],