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/FileChangeMonitor.h

@@ -111,7 +111,7 @@ class CFileChangeMonitorListener
 {
 public:
     CFileChangeMonitorListener()
-        : m_pMonitor(NULL)
+        : m_pMonitor(nullptr)
     {
     }