Tabris developer guide should contain more detailed information about the ScrollingComposite internals

The ScrollingComposite is a very helpful utility but it has some internal details that should be mentioned in the guide. Namely the duality of the ScrolledComposite and the actual (Scrolling-)Composite on top.

There are cases where you need to access the ScrolledComposte at the bottom (via getParent()), eg. when you use the ScrollingComposite as the content of a SwipeItem. Directly returning the ScrollingComposite in the load method of the swipe item crashes the clients.