[ticket/14387] Extend avatar-driver by extension in ACP not possible

Create a driver method to provide the driver config name, and use it
within the manager method. Default driver config name is the same as now.
But new drivers are able to override the config name with their own.

PHPBB3-14387

phpBB/phpbb/avatar/manager.php

@@ -276,7 +276,7 @@ class manager
 	*/
 	public function get_driver_config_name($driver)
 	{
-		return preg_replace('#^phpbb\\\\avatar\\\\driver\\\\#', '', get_class($driver));
+		return $driver->get_config_name();
 	}
 
 	/**