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/AboutDialog.cpp

@@ -25,7 +25,7 @@ AZ_PUSH_DISABLE_DLL_EXPORT_MEMBER_WARNING
 #include <ui_AboutDialog.h>
 AZ_POP_DISABLE_DLL_EXPORT_MEMBER_WARNING
 
-CAboutDialog::CAboutDialog(QString versionText, QString richTextCopyrightNotice, QWidget* pParent /*=NULL*/)
+CAboutDialog::CAboutDialog(QString versionText, QString richTextCopyrightNotice, QWidget* pParent /*=nullptr*/)
     : QDialog(pParent)
     , m_ui(new Ui::CAboutDialog)
 {