Fix crash when deleting all wxTreeListCtrl items with wxGTK3

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.

src/generic/treelist.cpp

@@ -572,12 +572,16 @@ void wxTreeListModel::DeleteItem(Node* item)
 
 void wxTreeListModel::DeleteAllItems()
 {
+    // Note that this must be called before actually deleting the items as
+    // clearing GTK+ wxDataViewCtrl results in SELECTION_CHANGED events being
+    // sent and these events contain pointers to the model items, so they must
+    // still be valid.
+    Cleared();
+
     while ( m_root->GetChild() )
     {
         m_root->DeleteChild();
     }
-
-    Cleared();
 }
 
 const wxString& wxTreeListModel::GetItemText(Node* item, unsigned col) const