Add "Open Quickstart Guide" and "FAQ" buttons to the Help menu

While we're at it, also refactor the function used by OnOpenModsPage to be compatible with other URLs

src/yuzu/main.h

@@ -181,6 +181,9 @@ private slots:
     void OnPauseGame();
     void OnStopGame();
     void OnMenuReportCompatibility();
+    void OnOpenModsPage();
+    void OnQuickstartGuide();
+    void OnFAQ();
     /// Called whenever a user selects a game in the game list widget.
     void OnGameListLoadFile(QString game_path);
     void OnGameListOpenFolder(GameListOpenTarget target, const std::string& game_path);
@@ -219,6 +222,7 @@ private:
     void UpdateStatusBar();
     void HideMouseCursor();
     void ShowMouseCursor();
+    void OpenURL(const QString& url_str);
 
     Ui::MainWindow ui;