Fix for assertion failure when displaying Add Property dialog on Linux

Assertion was "../src/gtk/dataview.cpp(3639): assert "parent_node" failed in
ItemAdded(): Did you forget a call to ItemAdded()? The parent node is unknown
to the wxGtkTreeModel"

I could find no logical reason for this error since ItemAdded() certainly WAS
being called for the parent item, however switching to the built-in
wxTreeListCtrl fixes the problem (at the expense of losing certain advanced
functionality like rich text formatting).