Add Open Achievements Hotkey

Adds a hotkey to pause emulation and bring up the Achievements dialog.

Source/Core/DolphinQt/HotkeyScheduler.h

@@ -53,6 +53,9 @@ signals:
   void ExportRecording();
   void ToggleReadOnlyMode();
   void ConnectWiiRemote(int id);
+#ifdef USE_RETRO_ACHIEVEMENTS
+  void OpenAchievements();
+#endif  // USE_RETRO_ACHIEVEMENTS
 
   void Step();
   void StepOver();