4.8.35-beta: Add new methods to FormElement class

• Add new methods to FormElement class

$formElement->required('Please inser First name');

// or

$formElement->addValidationMessage('required', ''Please inser First name');

// or

$formElement->setValidationMessages([
    'required', ''Please inser First name'
    'unique', ''First name exists!'
]);

• Fix datetimepicker
• Fix multiselect form input name