File entity is now a classed object

Backdrop now has a consistent way of performing all CRUD functions for entities, as well as accessing entity properties such as label, revision, id, etc.

This applies to:

Taxonomy Term (and Vocabulary) entities
User
Node
File

Example implementation:

Create a File entity:

$url = 'path/to/file.file_extension'; // Can be a stream wrapper, such as 'Public://myfile.txt'

$file = entity_create('file', array(
  'fid' => NULL,
  'uid' => 1,
  'filename' => 'newfile.file',
  'uri' => $url,
  'filemime' => 'text/plain',
  'filesize' => filesize($url),
  'status' => 1,
  'timestamp' => REQUEST_TIME,
));
$file->save();

Files entities are currently owned by System module. After completing this conversion, it's likely we will follow D8's suit here and move the hook_entity_info() for files into File module after this conversion.

Drupal.org issue: https://drupal.org/node/1361226