Store separate bindings for every different controller

[nmlgc]

This would mainly allow different controllers to be used without having to rebind every time you want to play with a different controller. But it would also help when using the same controller in a configuration that is used on both Windows and Linux, since Linux uses different IDs for the same buttons, and even seems to make up axes and buttons that physically don't exist. (At least Wine does.)

Internally, we would differentiate using the SDL-reported GUID, while we use the SDL-reported name in the Joy Pad menu.