Remove game.gameframerate as duplicate of game.slowdown

game.gameframerate seems to exist for converting the value of
game.slowdown into an actual timestep value, when really the timestep
value should just use game.slowdown directly with a fast lookup table.
Otherwise, there's a bunch of duplicated game.slowdown case-switches
everywhere, which adds up to a large, annoying pile should the values be
changed in the future. But now the duplicate variable has been removed,
and with it, all the copy-pasted case-switches.

Also, the game speed text rendering in Menu::accessibility and
Menu::setslowdown has been factored out to a function and de-duplicated
as well.

desktop_version/src/Game.h

@@ -275,7 +275,6 @@ public:
     bool  colourblindmode;
     bool noflashingmode;
     int slowdown;
-    Uint32 gameframerate;
 
     bool nodeathmode;
     int gameoverdelay;