GTK+ 3 (but not the generic version nor even GTK+ 2, apparently) sends
"selection changed" event from gtk_tree_model_row_deleted() when
deleting the currently selected row, which resulted in sending of
wxEVT_TREELIST_SELECTION_CHANGED events with invalid wxTreeListItem,
containing a dangling pointer, and a crash in the treelist sample when
trying to dump it.
Avoid this by clearing the model (and hence generating these events)
first and deleting the items only afterwards.
Also add a trivial unit test for wxTreeListCtrl::DeleteAllItems(), which
doesn't even allow to reproduce this bug, but is still probably better
to have than not to.
Closes#18045.