Added Change Media client functionality to AchievementMananger

The client can handle media changes natively so disabling can take place internally. This code uses the same external calls to load data, but will call either BeginLoad or BeginChangeMedia based on whether any media is already loaded.
Due to the client's handling of media changes (it simply disables hardcore if an unknown media is detected) the existing functionality for "disabling" the achievements is no longer necessary and can be deleted.

Source/Core/DolphinQt/Achievements/AchievementHeaderWidget.h

@@ -26,7 +26,6 @@ private:
   QLabel* m_points;
   QProgressBar* m_game_progress;
   QLabel* m_rich_presence;
-  QLabel* m_locked_warning;
   QGroupBox* m_header_box;
 };