Use enums for swngame

This replaces the swngame int variable with a named enum and enforces
strict typechecking on it.

Strict typechecking is okay here as the swngame variable is not part of
the API surface of the game in any way and is completely internal.

And just to make things clear, I've added a SWN_NONE enum to use for
initialization, because previously it was being initialized to 0, even
though 0 was the Gravitron.

desktop_version/src/Entity.cpp

@@ -1762,7 +1762,7 @@ void entityclass::createentity(int xp, int yp, int t, int meta1, int meta2, int
         entity.colour = 21;
         entity.tile = 78; //default case
         entity.animate = 1;
-        if (game.swngame == 1)
+        if (game.swngame == SWN_SUPERGRAVITRON)
         {
             //set colour based on current state
             entity.colour = swncolour(game.swncolstate);