Added OnElementNotFound system event

Allows a plugin to provide the element (chunk/snippet) like this:

```php
<?php
if($class == 'modChunk' && $name == 'test') {
  $chunk = $modx->newObject('modChunk');
  $chunk->set('name', $name);
  $chunk->set('snippet', 'Output of the chunk');
  $modx->event->_output = $chunk;
}
return;
```