Add new buttons supported by iOS 13

[MrStahlfelge]

The following buttons were added:

var buttonMenu: GCControllerButtonInput (since iOS 13, buttonStart)
var buttonOptions: GCControllerButtonInput? (since iOS 13, buttonBack)
var leftThumbstickButton: GCControllerButtonInput? (since iOS 12.1, leftStick)
var rightThumbstickButton: GCControllerButtonInput? (since iOS 12.1, right stick)

setPausedHandler is deprecated, should be removed for iOS 13 and handled by Start Button.