Browser: Allow system shortcuts to function

Before shortcuts like ALT-F4, etc did not work. This makes the window
ignore keydown events. In the future this may be altered to allow for
custom shortcuts from within ladybird?

Userland/Applications/Browser/BrowserWindow.cpp

@@ -718,6 +718,10 @@ void BrowserWindow::event(Core::Event& event)
     case GUI::Event::Resize:
         broadcast_window_size(static_cast<GUI::ResizeEvent&>(event).size());
         break;
+    case GUI::Event::KeyDown:
+        // FIXME: add support for hotkeys *within* ladybird
+        event.ignore();
+        break;
     default:
         break;
     }