Frontend: Prevent FileSystemWatcher from blocking UI thread

Instead of tying the QFileSystemWatcher to the GameList and updating in
the UI thread, this change moves it to the worker thread. Since it gets
deleted and recreated as part of the worker thread, this prevents it from
ever getting used from multiple threads (which is why it was originally
done on the UI thread)

src/citra_qt/game_list.h

@@ -85,10 +85,9 @@ private slots:
 private:
     void AddEntry(const QList<QStandardItem*>& entry_items);
     void ValidateEntry(const QModelIndex& item);
-    void DonePopulating();
+    void DonePopulating(QStringList watch_list);
 
     void PopupContextMenu(const QPoint& menu_location);
-    void UpdateWatcherList(const std::string& path, unsigned int recursion);
     void RefreshGameDirectory();
     bool containsAllWords(QString haystack, QString userinput);
 
@@ -98,5 +97,5 @@ private:
     QTreeView* tree_view = nullptr;
     QStandardItemModel* item_model = nullptr;
     GameListWorker* current_worker = nullptr;
-    QFileSystemWatcher watcher;
+    QFileSystemWatcher* watcher = nullptr;
 };