Add a "Next Tile" window

I've added a window that you would commonly see in Tetris games, it displays the value of the next tile that will drop. I had to make some slight changes to the GameManager and HTMLActuator to get it to work, it should all look pretty straightforward in the diff though. Let me know if you have any questions. I did my best to fit it into the design using existing styles.

PS - this is my first attempt at contributing to anything outside of my current apprenticeship program, let me know if I'm doing anything wrong/annoying. Thanks!