Add a class for keyboard input

Removes the windows-specific hack from sceCtrl
to read keyboard input and instead add a hack to
PPSSPPWindows.

The input mapping is still hardcoded and is the same
as it was in sceCtrl.

Add an std::list to WindowsHost that, curretly, contains
a hardcoded list of preferred `InputDevice`s. Devices
are tried in-order until one of them reports success.
Xinput is preferred over Keyboard. Keyboard always
returns success. Pointers are stored in the std::list
for polymorphism.

Change XinputDevice to report failure as soon as a
device can't be queried, instead of only a frame later.

Windows/XinputDevice.cpp

@@ -40,12 +40,13 @@ int XinputDevice::UpdateState() {
 		__CtrlSetAnalog(left.x, left.y);
 		this->prevState = state;
 		this->check_delay = 0;
+		return 0;
 	} else {
 		// wait check_delay frames before polling the controller again
 		this->gamepad_idx = -1;
 		this->check_delay = 100;
+		return -1;
 	}
-	return 0;
 }
 
 // We only filter the left stick since PSP has no analog triggers or right stick