will be automatically freed when it is deleted. Note that we don't use wxObject
as the base class for wxTreeItemData because the size of this class is
critical: in many applications, each tree leaf will have wxTreeItemData
-associated with it and number of leaves may be quite big.
+associated with it and the number of leaves may be quite big.
Also please note that because the objects of this class are deleted by the tree
using the operator {\tt delete}, they must always be allocated on the heap