@gedex is `plugin_dir_path` symlink-friendly? What about `WP_PLUGIN_DIR` + `dirname( __FILE__ )`?

Also, I'd suggest using static member variable for the class instead of yet another global constant, so:

```php
class Widget_Customizer {
    static $plugin_dir;
    /* ... */
    static setup() {
        /* ... */
        self::$plugin_dir = WP_PLUGIN_DIR  . DIRECTORY_SEPARATOR _ dirname( __FILE__ ); // or whatever
```