Refactoring of main menu related code

We need to make sure it's not stay active during gameplay.
Proper fix would require actually removing menu interface from GH, but then GH needs refactoring too.

client/mainmenu/CMainMenu.h

@@ -54,6 +54,7 @@ public:
 	void deactivate() override;
 
 	void switchToTab(size_t index);
+	void switchToTab(std::string name);
 };
 
 class CMenuEntry : public CIntObject
@@ -140,8 +141,6 @@ public:
 	void openCampaignScreen(std::string name);
 
 	static CMainMenu * create();
-	void removeFromGui();
-	static void showLoadingScreen(std::function<void()> loader);
 
 	static std::shared_ptr<CPicture> createPicture(const JsonNode & config);