BaseMailer: Use textLayout for text version of Email when possible

For the moment even if ```$textLayout``` file exists it doesn't used for rendering Text version of Email in this example: 
```php
Yii::$app->mail->compose('confirm');
```

Only this construction will use ```$textLayout``` for rendering Text version of Email:
```php
Yii::$app->mail->compose([
    'html' => 'confirm-html',
    'text' => 'confirm-text',
]);
```
-----------
The suggestion is:
When ```$textLayout``` file exists, use that template by default for generating Email Text Body, instead of ```$htmlLayout```.

The change provided for BaseMailer.php is just to show the point.

Thanks.