Common Errors

1. Forget to set game_window as parent for Square, so each Square becomes a new window.
2. Set main_window as parent of game_window, cause the chess board to be inside the login screen.
3. nullptr exceptions when setting the highlighted squares, because of using the static "highlighted" data member without first checking if it is nullptr.
4. Attempting to connect game_window signal to main_window slot in main_window constructor, before game_window is constructed, causing nullptr exception.