Activate the category tree

- Add role to get the category path CategoryPathRole
- We can add a category under another category
- We can remove the category and this remove category child and detach
the todo
- Fix rename, now we need rename also the path of child category
- Add dnd of category, we can dnd one or severals category in another
- Move Separator from TodoNode to CategoryManager, TodoNode should
accessible only by the todo*model and not by others objects.