Abstract returning-to-editor code to Game::returntoeditor()

This way I can easily move it around without moving around a bunch of
lines of code.

desktop_version/src/Game.h

@@ -385,6 +385,10 @@ public:
     int fadetomenudelay;
     bool fadetolab;
     int fadetolabdelay;
+
+#if !defined(NO_CUSTOM_LEVELS)
+    void returntoeditor();
+#endif
 };
 
 extern Game game;