Don't send EXPANDED in generic wxTreeCtrl if not really expanded

On demand expandable items can fail to be actually expanded when the
user attempts to expand them, but the generic version of wxTreeCtrl
still sent an EXPANDED event in this case, which wasn't useful and
differed from the native MSW version.

Fix this and only send the event if the item was really expanded.

Closes https://github.com/wxWidgets/wxWidgets/pull/1450

Closes #13886.

src/generic/treectlg.cpp

@@ -1886,8 +1886,16 @@ void wxGenericTreeCtrl::Expand(const wxTreeItemId& itemId)
         m_dirty = true;
     }
 
+    // Don't send EXPANDED event unconditionally: if this is an item for which
+    // SetItemHasChildren(true) had been called before, but no children have
+    // been added from the EXPANDING handler, we shouldn't consider the item to
+    // be really expanded.
+    wxTreeItemIdValue cookie;
+    if ( GetFirstChild(item, cookie).IsOk() )
+    {
         event.SetEventType(wxEVT_TREE_ITEM_EXPANDED);
         GetEventHandler()->ProcessEvent( event );
+    }
 }
 
 void wxGenericTreeCtrl::Collapse(const wxTreeItemId& itemId)