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/SettingsManagerDialog.cpp

@@ -84,7 +84,7 @@ void CSettingsManagerDialog::OnReadBtnClick()
         ui->m_layoutListBox->clear();
 
         TToolNamesMap toolNames;
-        XmlNodeRef dummyNode = NULL;
+        XmlNodeRef dummyNode = nullptr;
 
         GetIEditor()->GetSettingsManager()->GetMatchingLayoutNames(toolNames, dummyNode, m_importFileStr);