avoid ctrl_nhwindow-related impossible

Remove the impossible(), so that new functionality can be added
incrementally without requiring every window-port to be visited up front.

win/Qt/qt_bind.cpp

@@ -514,11 +514,13 @@ win_request_info *NetHackQtBind::qt_ctrl_nhwindow(
         return (win_request_info *) 0;
 
     switch(request) {
+    case set_mode:
+    case request_settings:
+        break;
     case set_menu_promptstyle:
         /* = wri->fromcore.menu_promptstyle; */
         break;
     default:
-        impossible("invalid request to ctrl_nhwindow: %d", request);
         break;
     }
     return wri;