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/Undo/Undo.h

@@ -116,7 +116,7 @@ public:
                 continue;
             }
 
-            if (m_undoObjects[i]->GetObjectName() == NULL)
+            if (m_undoObjects[i]->GetObjectName() == nullptr)
             {
                 continue;
             }
@@ -209,7 +209,7 @@ public:
 
     bool IsHaveUndo() const;
     bool IsHaveRedo() const;
-    
+
     void SetMaxUndoStep(int steps);
     int GetMaxUndoStep() const;