Add function Game::returntomenu()

When exiting from a game-gamestate which may have been entered through a
varying amount of menus, the solution is to not use Game::returnmenu(),
and to instead have a way to go back to a certain given menu.

desktop_version/src/Game.h

@@ -106,6 +106,7 @@ public:
     std::string timetstring(int t);
 
     void returnmenu();
+    void returntomenu(enum Menu::MenuName t);
     void  createmenu(enum Menu::MenuName t, bool samemenu = false);
 
     void lifesequence();