Change class hierarchy of MenuTable::BaseItem...

... There are now four immediate subclasses (SharedItem and Computed Item,
which are final, and SingleItem and GroupItem, which are abstract), which may
serve future purposes more general than menu items.  There are further
subclasses specific to menu management.

The former concrete class GroupItem is renamed TransparentGroupItem.

Also allows direct construction of items in lists from shared pointers.

src/Menus.cpp

@@ -104,8 +104,12 @@ namespace MenuTable {
 
 BaseItem::~BaseItem() {}
 
+SharedItem::~SharedItem() {}
+
 ComputedItem::~ComputedItem() {}
 
+SingleItem::~SingleItem() {}
+
 GroupItem::GroupItem( BaseItemPtrs &&items_ )
 : items{ std::move( items_ ) }
 {
@@ -116,6 +120,11 @@ void GroupItem::AppendOne( BaseItemPtr&& ptr )
 }
 GroupItem::~GroupItem() {}
 
+TransparentGroupItem::~TransparentGroupItem() {}
+}
+
+namespace MenuTable {
+
 MenuItem::MenuItem( const TranslatableString &title_, BaseItemPtrs &&items_ )
 : GroupItem{ std::move( items_ ) }, title{ title_ }
 {
@@ -188,6 +197,12 @@ void VisitItem( AudacityProject &project, MenuTable::BaseItem *pItem )
    auto &manager = CommandManager::Get( project );
 
    using namespace MenuTable;
+   if (const auto pShared =
+       dynamic_cast<SharedItem*>( pItem )) {
+      auto delegate = pShared->ptr;
+      VisitItem( project, delegate.get() );
+   }
+   else
    if (const auto pComputed =
        dynamic_cast<ComputedItem*>( pItem )) {
       // TODO maybe?  memo-ize the results of the function, but that requires
@@ -235,7 +250,7 @@ void VisitItem( AudacityProject &project, MenuTable::BaseItem *pItem )
    }
    else
    if (const auto pGroup =
-       dynamic_cast<GroupItem*>( pItem )) {
+       dynamic_cast<TransparentGroupItem*>( pItem )) {
       // recursion
       VisitItems( project, pGroup->items );
    }