Editor code: tidy up BOOLs,NULLs and overrides pt4.

A few 'typedefs' replaced by 'using's
This shouldn't have any functional changes at all, just c++17 modernization
It's a part 4 of a split #2847

Signed-off-by: Nemerle <nemerle5+git@gmail.com>

Code/Editor/Util/IndexedFiles.h

@@ -88,7 +88,7 @@ public:
     }
 
 public:
-    void Initialize(const QString& path, IFileUtil::ScanDirectoryUpdateCallBack updateCB = NULL);
+    void Initialize(const QString& path, IFileUtil::ScanDirectoryUpdateCallBack updateCB = nullptr);
 
     // Adds a new file to the database.
     void AddFile(const IFileUtil::FileDesc& path);