ENHANCEMENT: Allow programmers to set OwnerID for new files.

**NOTE:** This change should be reviewed to make sure it does not cause any side effects.

**Example use case:** An admin porting user images from an old website. 
The script would put the images in the assets folder, and then insert them into the database using Image::write() and data from the old database.

```php
public function insertImage($data) {
    $image = new Image();
    $image->ParentID = $data->parentId;
    $image->Title = $data->title;
    $image->FileName = $data->filename;
    $image->OwnerID = $data->ownerId;
    $image->write();
    
    // In the current version, this results in all images
    // being owned by Member::currentUser() instead of 
    // the expected $data->ownerId;
}
```