Multiplayer, JSR 305, and Code Cleanup

Overview

• Synchronized menu interaction between all viewers
• @Redrield added JSR 305 annotations. (Thanks!)
• Added support for java.util.Optional in State
• Cleaned up the code quite a bit

Breaking Changes:

• Methods for getting the values in State have been changed to be more conventional:
  • current() -> getCurrent()
  • previous() -> getPrevious()
  • Doesn't exist -> getOptionalCurrent()
  • Doesn't exist -> getOptionalPrevious()