Added support for getting the real controller info, as well as the function SDL_GameControllerGetSteamHandle() to get the Steam Input API handle, from the virtual gamepads provided by Steam.
Also added an event SDL_CONTROLLERSTEAMHANDLEUPDATED which is triggered when a controller's API handle changes, e.g. the controllers were reassigned slots in the Steam UI.
(cherry picked from commit c981a597dc)
