Change "toggle letterbox" to "scaling mode"

This change was half-backported from the localization branch, except I
just came up with "scaling mode" as a better term than the more generic
"graphics mode". It doesn't make sense to still have the option be
called "toggle letterbox" because a third option (integer mode) was
added at some point.

desktop_version/src/Game.cpp

@@ -7160,7 +7160,7 @@ void Game::createmenu( enum Menu::MenuName t, bool samemenu/*= false*/ )
         break;
     case Menu::graphicoptions:
         option("toggle fullscreen");
-        option("toggle letterbox");
+        option("scaling mode");
         option("resize to nearest", graphics.screenbuffer->isWindowed);
         option("toggle filter");
         option("toggle analogue");