Extract the root-node creation into its own method

`WidgyField.pre_save` creates a root node. This patch extracts that
creation into a method called `add_root`. This means

  - VersionedWidgyField no longer needs to override `pre_save`, it
    overrides `add_root`.
  - Application developers can call `add_root` themselves and provide
    root_content_kwargs when they need to provide values to create the
    root node (Like when it has non-null fields).