Gaphor use cases

An new element is added to a diagram:
- A new element is created
- (opt.) The namespace is set to the diagrams namespace
- A new item is created on the canvas with the element as subject
  * A refernce is created from the item to the element
  * a weak reference is created from the element to the item
  * a notifier is connected to signal changes on the element.

Remove an object:
- Removing can only be done through the diagram interface.
- The item is removed from the Element.presentation list
- If len(element.presentation) == 0:
   element.unlink()
   del element

