Added AchievementsWindow QDialog

AchievementsWindow is the dialog box that will eventually contain the settings and progress data for RetroAchievements on Dolphin. This adds the barebones dialog, and connects it to MainWindow's MenuBar.

Source/Core/DolphinQt/MenuBar.h

@@ -93,6 +93,10 @@ signals:
   void ShowInfinityBase();
   void ConnectWiiRemote(int id);
 
+#ifdef USE_RETRO_ACHIEVEMENTS
+  void ShowAchievementsWindow();
+#endif  // USE_RETRO_ACHIEVEMENTS
+
   // Options
   void Configure();
   void ConfigureGraphics();