Moving addons system and theme folders

Using NSM TinyMCE on a site of mine.
Got into the habit of moving my third party and theme/third_part folders recently. NSM Tiny MCE stopped working. Went in and changed a few things in  ft.nsm_tiny_mce.php:

* 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'] overrides

This is my first pull request ever and I am a front end dev so ... have a good look at it before merging ;o)

Also, thanks for that add-on. Still better than the default RTE IMHO.

Signed-off-by: Jérôme Coupé <jercoupe@gmail.com>