Qt: Set focus to display window when starting

src/duckstation-qt/mainwindow.h

@@ -50,6 +50,7 @@ private:
 
   std::unique_ptr<GameList> m_game_list;
   GameListWidget* m_game_list_widget = nullptr;
+  QWidget* m_display_widget = nullptr;
 
   SettingsDialog* m_settings_dialog = nullptr;