left-handed mode for OneSaber

[digama0]

When playing OneSaber maps, this allows choosing to use the left or right hand based on the active controller used to select the "PLAY" or "RESTART GAME" button. The active handedness cannot be changed during the game, since I thought that might make it not really "one saber" anymore. Moreover, the notes, obstacles and events are all left-right mirrored in left-handed mode. I believe this matches the behavior of Beat Saber's handedness setting.