Fix #380

Achievements could be unlocked in custom levels/make and play, so this adds the wrapper function `game.unlockAchievement` which calls `NETWORK_unlockAchievement` if `map.custommode` is false.
Also, this function and `Game::unlocknum` have both been `ifdef`ed to be empty if MAKEANDPLAY is defined.

desktop_version/src/Game.h

@@ -413,6 +413,7 @@ public:
 
     bool shouldreturntopausemenu;
     void returntopausemenu();
+	void unlockAchievement(const char *name);
 
     bool disablepause;
 };