Add a helper function to get the last tree item to the sample.

This makes the behaviour of different menu commands working with the "last
item" consistent as some of them used the last root child while others used
the really last item (i.e. the last child of the last child).

This should have been part of r75987.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76013 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/treectrl/treetest.h

@@ -94,6 +94,7 @@ public:
     void OnRMouseUp(wxMouseEvent& event);
     void OnRMouseDClick(wxMouseEvent& event);
 
+    wxTreeItemId GetLastTreeITem() const;
     void GetItemsRecursively(const wxTreeItemId& idParent,
                              wxTreeItemIdValue cookie = 0);
 
@@ -137,6 +138,7 @@ protected:
     }
 
 private:
+    // Find the very last item in the tree.
     void AddItemsRecursively(const wxTreeItemId& idParent,
                              size_t nChildren,
                              size_t depth,