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/CheckOutDialog.h

@@ -34,7 +34,7 @@ public:
         CANCEL = QDialog::Rejected
     };
 
-    CCheckOutDialog(const QString& file, QWidget* pParent = NULL);   // standard constructor
+    CCheckOutDialog(const QString& file, QWidget* pParent = nullptr);   // standard constructor
     virtual ~CCheckOutDialog();
 
     // Dialog Data