Fixed issue that when go back to start screen, start view doesn't fit

in to the decorator properly.

- Moved start screen view initialization logic into ViewManager.
- When go back to start screen view, a new stage within the start screen
view will be initialized.

#story[1245]

src/main/java/seng302/visualiser/controllers/LobbyController.java

@@ -108,7 +108,6 @@ public class LobbyController implements Initializable {
 
     public void leaveLobby() {
         // TODO: 10/07/17 wmu16 - Finish function!
-        GameState.setCurrentStage(GameStages.CANCELLED);
 //        for (LobbyController_old.LobbyCloseListener readyListener : lobbyListeners)
 //            readyListener.notify(LobbyController_old.CloseStatus.LEAVE);
         ViewManager.getInstance().getGameClient().stopGame();