AchievementManager: Use HookableEvent instead of std::function callbacks

The callback mechanism AchievementManager had until now only supported
one caller registering a callback, and it didn't have any
synchronization. This isn't a problem for DolphinQt, but the PR to add
Android support for RetroAchievements exposes these problems. Let's
replace it with HookableEvent, which can handle all of this.

Source/Core/DolphinQt/Achievements/AchievementsWindow.cpp

@@ -30,12 +30,16 @@ AchievementsWindow::AchievementsWindow(QWidget* parent) : QDialog(parent)
 
   CreateMainLayout();
   ConnectWidgets();
-  AchievementManager::GetInstance().SetUpdateCallback(
+
+  m_event_hook = AchievementManager::UpdateEvent::Register(
       [this](AchievementManager::UpdatedItems updated_items) {
         QueueOnObject(this, [this, updated_items = std::move(updated_items)] {
           AchievementsWindow::UpdateData(std::move(updated_items));
         });
-      });
+      },
+      "AchievementsWindow");
+  UpdateData(AchievementManager::UpdatedItems{.all = true});
+
   connect(&Settings::Instance(), &Settings::EmulationStateChanged, this,
           [this] { m_settings_widget->UpdateData(RC_OK); });
 }