[PR] Allow replacing xml form field's descendants

This PR addresses the issue of replacing XML for fields definitions which contain descendants like <option> nodes for list field.

Lets consider a content plugin with the following implementation:
 
```php
public function onContentPrepareForm($form, $data)
{
    // The name of the form we want to process
    $formName = 'com_config.application';
 
    // The xml definition of our additions/replacements
    $fileName = __DIR__.'/forms/'.$formName.'.xml';
 
    // Only process com_config application forms and only process it if the override file exists
    if ($form->getName() == $formName && is_file($fileName))
    {
        // Attempt to load the XML file as a simpleXml object
        $gaeXml = simplexml_load_file($fileName);
 
        // Replace XML nodes[aka Joomla Form fields] in this form with ones that have been defined in our replacement file
        $form->load($gaeXml);
    }
}
```

Basically, its intention is to add/replace elements from a form,
Joomla 'Global Configuration' form in this case


#### Extracted, original definition of a form field from com_config.application

```xml
<field
  name="mailer"
  type="list"
  default="mail"
  label="COM_CONFIG_FIELD_MAIL_MAILER_LABEL"
  description="COM_CONFIG_FIELD_MAIL_MAILER_DESC"
  required="true"
  filter="word">
    <option value="mail">COM_CONFIG_FIELD_VALUE_PHP_MAIL</option>
    <option value="sendmail">COM_CONFIG_FIELD_VALUE_SENDMAIL</option>
    <option value="smtp">COM_CONFIG_FIELD_VALUE_SMTP</option>
</field>
```

#### Field definition we want to set instead and also the expected result
(added 'gae' option and changed 'default' attribute)

```xml
<field
  name="mailer"
  type="list"
  default="gae"
  label="COM_CONFIG_FIELD_MAIL_MAILER_LABEL"
  description="COM_CONFIG_FIELD_MAIL_MAILER_DESC"
  required="true"
  filter="word">
    <option value="gae">COM_CONFIG_FIELD_VALUE_GAE_MAIL</option>
    <option value="mail">COM_CONFIG_FIELD_VALUE_PHP_MAIL</option>
    <option value="sendmail">COM_CONFIG_FIELD_VALUE_SENDMAIL</option>
    <option value="smtp">COM_CONFIG_FIELD_VALUE_SMTP</option>
</field>
```

#### Actual result is as follows (the descendants options are not copied over):

 ```xml
<field
  name="mailer"
  type="list"
  default="gae"
  label="COM_CONFIG_FIELD_MAIL_MAILER_LABEL"
  description="COM_CONFIG_FIELD_MAIL_MAILER_DESC"
  required="true"
  filter="word">
</field>
```

I think, when replacing a form this way, all nodes should be copied.