Adds an alias to the in-memory alias table for this object.
type $path: The system path of the alias.
type $alias: The alias of the system path.
type $path_language: The language of this alias.
public function addAlias($path, $alias, $path_language = NULL) {
$language = $path_language ?: $this->defaultLanguage;
$this->aliases[$path][$language] = $alias;
$this->systemPaths[$alias][$language] = $path;
}