Moving the addons theme and system folders

* used PATH_THIRD_THEMES in the constructor
* used URL_THIRD_THEMES in _getThemeUrl

Tested with files and folders in their default locations and nothing in config.php. Tested with addons moved in a custom location with $config['third_party_path'], $config['path_third_themes'] and $config['url_third_themes']

Signed-off-by: Jérôme Coupé <jercoupe@gmail.com>