Use enums for most of the rest of the game constants.

This adds an "enum map" type that allows safe usage of an enum to index an array.
(That is, it enforces that the index is of the enum type.)

src/game/boe.townspec.cpp

@@ -20,7 +20,7 @@
 #include "utility.hpp"
 
 extern eGameMode overall_mode;
-extern short stat_window;
+extern eItemWinMode stat_window;
 extern location center;
 extern sf::RenderWindow mainPtr;
 extern cUniverse univ;