[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.
Someone (archshift, Armada?) said they wanted to rewrite the whole thing anyways to allow better support for controllers like the Hydra. Their idea was to have a base class for the lower level stuff (which works on a BT level) and another one at higher level (using command messages). One of them would be used for Real Wiimote while the other would be used for emulated (and possibly the Hydra).
At least that would keep things a bit better seperated with just a layer translating from the low level to high level.