Controls update

This update aims to improve controls for non-mouse users:

• Touch Joystick: Added a basic directional-pad style control for touchscreen users who may prefer this over swiping. I don't expect many people to use this but it was easy to add so I did it ^.^  I feel that it could be improved, perhaps with movement indicators that appear around the paddle? Let me know your thoughts if you actually use this!
• Bigger Offset: Changed the "paddle offset" option to be more sensible. Instead of a value from 0-10, the options are No Offset, 10px, 20px, and 30px. (again for touchscreen, to avoid covering the paddle with your finger)
• Slowdown Button: Added a new keyboard control (default Z). Pressing this cuts the paddle movement speed in half for better accuracy.

As always don't hesitate to voice your suggestions - it is my goal to provide good controls regardless of whether you're using mouse, keyboard or touchscreen!