Desktop Gaming

This adds support for launching desktop games.

To add desktop games to Kart, you should create directories like you do for different emulated consoles, and drag a `LINK` to the executable into them. You should use symlinks (OS X) or shortcuts (Windows) in the appropriate directories. All links, Windows and OS X, should have the extension `.lnk` on them for them to be seen by Kart. The normal conventions apply for art.

### Directories
* Mac games - `/mac`
* Windows games - `/pc`