Add missing import to fix build

[henryalias]

The project runs fine in the Editor but fails to build because of a missing import. I fixed this here.

The code that was breaking (CommandManager:145):

#if UNITY_EDITOR
        UnityEditor.EditorApplication.isPlaying = false;
#else
        Console.LogWarning("pepehands");
        Application.Quit(); <-- here
#endif