Adding gamepad support

The player can use the keyboard to play our game, but it would be nice to also allow playing it using a gamepad. Fortunately, Qt provides the Qt Gamepad add-on that allows us to do this easily. As opposed to Qt Essentials (for example, Qt Widgets), add-ons may be supported on a limited number of platforms. As of Qt 5.9, Qt Gamepad supports Windows, Linux, Android, macOS, iOS, and tvOS (including the tvOS remote).