Editor code: tidy up BOOLs,NULLs and overrides pt6.

A few 'typedefs' replaced by 'using's
This shouldn't have any functional changes at all, just c++17 modernization
It's a part 6 of a split #2847

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

Code/Editor/LevelInfo.cpp

@@ -93,7 +93,7 @@ void CLevelInfo::ValidateObjects()
 
         pObject->Validate(m_pReport);
 
-        m_pReport->SetCurrentValidatorObject(NULL);
+        m_pReport->SetCurrentValidatorObject(nullptr);
     }
 
     CLogFile::WriteLine("Validating Duplicate Objects...");