About composite items:


- The owner of a composite item should implement the DiaCanvasGroupable interface
- a composite item can hold other items, these items should also be composite!
- if a composite item is selected, the non-composite parent is selected instead
- if a composite item gets the focus, it keeps the focus though (for example
  a text field in some object)
- same for grabbed items.
- signals are emited when the selection state changes for an item.
