Added Disabled flag to Achievement Manager

The Disabled state sits between Game Closed and completely Shutdown - stronger than the former, as it refuses to let a game be opened again until AchievementManager is restored (which only happens upon a fresh core boot) but it isn't completely shut down and will still allow the player to be logged in and access the achievement settings and their (global) achievement header.

Source/Core/Core/BootManager.cpp

@@ -164,6 +164,10 @@ bool BootCore(std::unique_ptr<BootParameters> boot, const WindowSystemInfo& wsi)
     }
   }
 
+#ifdef USE_RETRO_ACHIEVEMENTS
+  AchievementManager::GetInstance()->SetDisabled(false);
+#endif  // USE_RETRO_ACHIEVEMENTS
+
   const bool load_ipl = !StartUp.bWii && !Config::Get(Config::MAIN_SKIP_IPL) &&
                         std::holds_alternative<BootParameters::Disc>(boot->parameters);
   if (load_ipl)