Added GameCube Adapter support.

The libusb driver must be installed on the adapter (e.g. zadig can be used to install the driver in Windows).  GameCube pad controllers are supported and will override the current input device assigned to the port.  GameCube controller buttons are auto-configured and cannot be re-assigned.  Rumble is supported.  Hotplug is supported while playing a game.  If a controller is unplugged from the adapter, Dolphin will fallback to using the host input device on that port.  If a port on the adapter is unused, Dolphin will use the host input device for that port, allowing a mixture of host input devices and controllers connected to the adapter.

The adapter support can be disabled in the Controllers config if the OS driver is preferred (allowing the pad buttons to be reconfigured).

One adapter per system is supported.
Retrieved the endpoint addresses of the GC Adapter via the descriptor.
Fixed wireless GC controller detection.
Made the controller update thread-safe.
Added pad origin to wired and wireless pads only.
Moved the GC Adapter processing under "Standard Controller" as it appears the adapter does not work with controllers other than the standard controller (pads, bongos, some dance mats).
Merge pull request #1678 from skidau/GC-Adapter

Added GameCube Adapter support