Editor code: tidy up BOOLs,NULLs and overrides pt5. (#2876)

A few 'typedefs' replaced by 'using's
This shouldn't have any functional changes at all, just c++17 modernization
It's a part 5 of a split #2847

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

Co-authored-by: Nemerle <nemerle5+git@gmail.com>

Code/Editor/EditorFileMonitor.h

@@ -42,7 +42,7 @@ private:
         QString extension;
 
         SFileChangeCallback()
-            : pListener(NULL)
+            : pListener(nullptr)
         {}
 
         SFileChangeCallback(IFileChangeListener* pListener, const char* item, const char* extension)