[game_list] fix crash and flashing from the directory watcher (#4099)

So the game list watcher could rebuild the content providers
(CreateFactories) while the populate worker was still scanning them, and
the worker would walk a torn-down RegisteredCache and segfault in
OpenFileOrDirectoryConcat. Fixed by stopping and joining the worker
before rebuilding.

On macOS the watcher also re-armed itself every populate. Re-adding the
same paths makes QFileSystemWatcher re-emit directoryChanged (the
FSEvent comes in async, so the blockSignals guard misses it), so it just
kept refreshing and the list flashed forever. Now it only re-arms when
the watched dirs actually changed.

Also null-guarded OpenFileOrDirectoryConcat so a torn-down cache cant
null-deref there.

Related: https://github.com/eden-emulator/Issue-Reports/issues/336
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4099
Reviewed-by: Lizzie <lizzie@eden-emu.dev>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>

src/qt_common/game_list/model.h

@@ -52,6 +52,10 @@ public:
     void DonePopulating(const QStringList& watch_list);
 
     void PopulateAsync(QVector<UISettings::GameDir>& game_dirs);
+    // Stops and joins the running populate worker, if any. Must be called before rebuilding the
+    // content providers (CreateFactories), otherwise the worker keeps scanning a cache that is
+    // being torn down underneath it.
+    void StopWorker();
     void WorkerEvent();
 
     bool IsEmpty() const;