Use QFileSystemWatcher to reload the game list when a change is detected. (#2555)

* Added a refresh game directory option to the file menu

* Make the game list watcher recursive and have it start watching from the initial load

* Rework game list watcher to be thread safe

* Fix code style issues

src/citra_qt/game_list.h

@@ -4,6 +4,7 @@
 
 #pragma once
 
+#include <QFileSystemWatcher>
 #include <QModelIndex>
 #include <QSettings>
 #include <QStandardItem>
@@ -46,8 +47,11 @@ private:
     void DonePopulating();
 
     void PopupContextMenu(const QPoint& menu_location);
+    void UpdateWatcherList(const std::string& path, unsigned int recursion);
+    void RefreshGameDirectory();
 
     QTreeView* tree_view = nullptr;
     QStandardItemModel* item_model = nullptr;
     GameListWorker* current_worker = nullptr;
+    QFileSystemWatcher watcher;
 };