Use helper class instead of manually calling gtk_tree_path_free().

Use helper class instead of manually calling gtk_tree_path_free().

No real changes, just simplify the code and make it more obviously correct by
ensuring that all temporary tree paths are always destroyed because they are
assigned to a wxGtkTreePath object which does it in its dtor instead of
calling gtk_tree_path_free() manually every time.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65212 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775