Use dynamic credits lists for main menu credits screens (and add GitHub contributors) (#160)

Also added GitHub contributors to the credits on the main menu
(previously they were only present in the credits at the end of the
game)

desktop_version/src/Game.h

@@ -184,6 +184,7 @@ public:
     std::vector<bool> menuoptionsactive;
     int nummenuoptions, currentmenuoption ;
     std::string menuselection, currentmenuname, previousmenuname;
+    int current_credits_list_index;
     int menuxoff, menuyoff;
 
     int menucountdown;