Made wxList::compatibility_iterator a class in wxUSE_STL == 0 case too instead

Made wxList::compatibility_iterator a class in wxUSE_STL == 0 case too instead
of a simple typedef to "Node *" to be able to initialize it with NULL in ctor
which makes the behaviour of the code with and without wxUSE_STL the same.

The price to pay is that the code like

	wxList:compatibility_iterator ci;
	ci = cond ? list->GetFirst() : wxList::compatibility_iterator();

doesn't compile any more and has to be replaced with (more clear anyhow)

	wxList:compatibility_iterator ci;
	if ( cond )
		ci = list->GetFirst();

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@38247 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775