Common: Make recursive FileUtil functions take a maximum recursion

Fixes #1115.

Also improves the performances of DiskArchive’s directory
implementation a lot, simply by not going through the entire tree
instead of just listing the first level files.

Thanks to JayRoxFox for rebasing this on current master!

src/citra_qt/game_list_p.h

@@ -212,5 +212,5 @@ private:
     bool deep_scan;
     std::atomic_bool stop_processing;
 
-    void AddFstEntriesToGameList(const std::string& dir_path, bool deep_scan);
+    void AddFstEntriesToGameList(const std::string& dir_path, unsigned int recursion = 0);
 };