Added cheat manager GUI.

**NOTE: Cheats don't actually work yet. This is just the GUI.**

Cheats are now accessible via the menu button. It supports Gameshark v1
and v3.
To hook them into the game, use GBACheatIOFramework and the GBACheat
class. GBACheat is pretty self explanatory, and GBACheatIOFramework is
commented.

DM me on Twitter if you have any questions.