Set given key value, trying to preserve previous key value too. This is useful for safely setting X3D META values.
Compared to normal PutKeyData, this behaves smarter if given Name is already set. If it's set with the same Content, we do nothing. If the Content is different, we move previous content to a Name + '-previous' key. This way previous content value is preserved once (but not more, to not grow the X3D file indefinitely).