Editor code: tidy up BOOLs,NULLs and overrides pt2.

A few 'typedefs' replaced by 'using's
This shouldn't have any functional changes at all, just c++17 modernization
It's a part 2 of a split #2847

Signed-off-by: Nemerle <nemerle5+git@gmail.com>

Code/Editor/Objects/SelectionGroup.cpp

@@ -636,7 +636,7 @@ void CSelectionGroup::FinishChanges()
     for (int i = 0; i < iObjectSize; ++i)
     {
         CBaseObject* pObject = selectedObjects[i];
-        if (pObject == NULL)
+        if (pObject == nullptr)
         {
             continue;
         }