Support additional controller buttons

The engine currently only supports an "XBox 360/One"-style controller with about 16 buttons: four D-pad directions, six other main face buttons (A/B/X/Y/Menu/Back), two shoulder buttons, two triggers, and two thumbstick press-down switches. However, many newer controllers (Steam Deck, 8BitDo Ultimate, etc.) offer additional triggers, shoulder buttons, back paddles, and/or extra face buttons, which could be helpful in a game with a complex UI like openmw. (One obvious indicator of a problem is that the controller configuration menu contains many more bindable actions than there are recognized controller buttons.)

I can't think of any good reason not to support all of the buttons defined at https://wiki.libsdl.org/SDL2/SDL_GameControllerButton, which includes all of the above plus Guide, Misc, Touchpad, and four paddles; that still wouldn't cover everything on the Deck or Ultimate, but it's almost a 50% enhancement over the current support.