Consistently make CaptureConfig a class.  Fixes a compiler warning.
Port the application to OS X using Qt and Cocoa, without using X11.

 * Calling XFixesGetCursorImage on OS X produces a stock arrow cursor
   centered in the screen.  (i.e. It's not really implemented.)
   Instead, use [NSCursor currentScreenCursor] to get the cursor's image
   and hotspot location.

 * Use [NSEvent mouseLocation] instead of XFixesGetCursorImage.

 * Use [NSEvent modifierFlags] to check the CAPSLOCK state.

I'm not sure this code works if the user has multiple screens, or if one
of the screens is a Retina display.  I don't have access to a Retina
display.