[WIP] Wiimote: use inheritance instead of #ifdef

The OS X implementation doesn't compile yet. Could someone familiar with Objective C please help me find a good way to solve this, @comex maybe? My primitive solution would be making the class variables public and casting the objects from the `g_wiimotes` list to `WiimoteDarwin` but I'm sure this can be done more elegantly.
Haven't looked into it, yet.