Add an extremely simple RAII wrapper around GList and use it.
Also add wxGtkTreePathList which also automatically frees its contents to
simplify working with the lists of GtkTreePaths.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68842 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775