Convert menu names to be an enum instead of being stringly-typed

Stringly-typed things are bad, because if you make a typo when typing
out a string, it's not caught at compile-time. And in the case of this
menu system, you'd have to do an excessive amount of testing to uncover
any bugs caused by a typo. Why do that when you can just use an enum and
catch compile-time errors instead?

Also, you can't use switch-case statements on stringly-typed variables.

So every menu name is now in the enum Menu::MenuName, but you can simply
refer to a menu name by just prefixing it with Menu::.

Unfortunately, I've had to change the "continue" menu name to be
"continuemenu", because "continue" is a keyword in C and C++. Also, it
looks like "timetrialcomplete4" is an unused menu name, even though it
was referenced in Render.cpp.

desktop_version/src/Script.cpp

@@ -2534,7 +2534,7 @@ void scriptclass::resetgametomenu()
 	graphics.flipmode = false;
 	obj.entities.clear();
 	graphics.fademode = 4;
-	game.createmenu("gameover");
+	game.createmenu(Menu::gameover);
 }
 
 void scriptclass::startgamemode( int t )