Define movable_ptr and make_movable, use __AUDACITY_OLD_STD__ in fewer places

src/UndoManager.cpp

@@ -265,17 +265,10 @@ void UndoManager::PushState(const TrackList * l,
 
    // Assume tags was duplicted before any changes.
    // Just save a new shared_ptr to it.
-#ifdef __AUDACITY_OLD_STD__
    stack.push_back(
-      std::make_shared<UndoStackElem>
+      make_movable<UndoStackElem>
          (std::move(tracksCopy),
             longDescription, shortDescription, selectedRegion, tags)
-#else
-   stack.emplace_back(
-      std::make_unique<UndoStackElem>
-         (std::move(tracksCopy),
-            longDescription, shortDescription, selectedRegion, tags)
-#endif
    );
 
    current++;