Refactored state machine.

Added a state info map which keeps track of a particular state's on_init, on_update and on_exit functions.
A few of the on_update functions have sub-states, which create extremely long switch statements.
Not sure if perhaps the best route would be to keep the switch statements but break the cases into individual
functions or if we should keep track of substates. The latter sounds like it could get complicated and really
messy quick. Problem for another day.

include/splash_screen.h

@@ -8,6 +8,6 @@
 #define SPLASH_DURATION_FRAMES (SPLASH_FPS * SPLASH_DURATION_SECONDS)
 
 void splash_screen_init();
-void splash_screen_update(uint timer);
+void splash_screen_update();
 
-#endif // SPLASH_SCREEN_H
+#endif // SPLASH_SCREEN_H