It wasn't detecting that the window was closed when I closed it by
clicking the red X button. As a result, the window would close, but
mfb_update() didn't return -1, so the program kept running.
This was fixed by listening to the willClose event and using it to set a
flag. I'm not sure if this is the best way to do it, I really know
nothing about OSX/Cocoa.
Also, I fixed a few unused varible warnings. And warnings related to
NSUInteger not matching the expected signature with NSWindowStyleMask.
