Changed GameGUI to be Singleton

Created MainMenu Singleton Class

MainMenu starts with just a button to load DesertUprising
 (plus a picture and a logo)

Now jumps back to MainMenu when you lose. Will change later to make more
obvious

Waves are currently not working. I can't figure out where to call
startWaves without getting a bunch of errors. Think this has something
to do with us not having it in sync with the ticker yet