Set GamePadState to partial to allow platform-specific extensions.

Each platform's controller has some unique features allowing some additional information on a per-player basis. While there's some overlap among them (accelerometers in the Wii and PS4 for instance), there are always going to be some inputs that are specific to a platform that can't be mimicked on others.

With this change the structure itself can be extended, meaning you don't have do additional GamePad(PlayerIndex) queries to get a platform-specific feature.