Added Steam Input API support for game controllers

Added support for getting the real controller info, as well as the function SDL_GetGamepadSteamHandle() to get the Steam Input API handle, from the virtual gamepads provided by Steam.

Also added an event SDL_EVENT_GAMEPAD_STEAM_HANDLE_UPDATED which is triggered when a controller's API handle changes, e.g. the controllers were reassigned slots in the Steam UI.

src/joystick/windows/SDL_windowsjoystick_c.h

@@ -42,6 +42,7 @@ typedef struct JoyStick_DeviceData
     Uint8 XInputUserId;
     DIDEVICEINSTANCE dxdevice;
     char path[MAX_PATH];
+    int steam_virtual_gamepad_slot;
     struct JoyStick_DeviceData *pNext;
 } JoyStick_DeviceData;