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/StringDlg.h

@@ -25,7 +25,7 @@ typedef bool (StringDlgPredicate)(QString input);
 class StringDlg : public QInputDialog
 {
 public:
-    StringDlg(const QString &title, QWidget* pParent = NULL, bool bFileNameLimitation = false);
+    StringDlg(const QString &title, QWidget* pParent = nullptr, bool bFileNameLimitation = false);
 
     void SetCheckCallback(const std::function<StringDlgPredicate>& Check) {
         m_Check = Check;