Apply a bodge to arrange that if the user selects Custom from the game

type dropdown, we still get an 'onchange' event if they select it a
second time. Normally this wouldn't happen, because onchange means
what it says and we only get it if a _different_ element is selected.

My solution is to create two list items called Custom, set one of them
as display:none to stop it showing up when the list is dropped down,
and to select it after the configuration box closes.

[originally from svn r9788]

emcc.c

@@ -723,7 +723,7 @@ int main(int argc, char **argv)
         js_add_preset(name);
     }
     if (thegame.can_configure)
-        js_add_preset("Custom");
+        js_add_preset(NULL);           /* the 'Custom' entry in the dropdown */
     else if (custom_preset == 0)
         js_remove_type_dropdown();