filesystem: add mac os x directory watcher this code is incredibly stinky tbh we should probably be using C++ threads everywhere else just because they are SO much easier to code for than the shitty Qt threads API

These icons are from Yusuke Kamiyamane's Fugue Icons[1] pack.
There are no changes divergent from any of the original files.

[1]: https://p.yusukekamiyamane.com/