CanvasItemModelSimple is used as a base class for the standard canvas item models.
It can also be used as the base class for new custom canvas item models.
<note><para> The Model/View canvas feature may be removed in a future version of GooCanvas. </para></note>
It provides default implementations for many of the CanvasItemModel methods.
Subclasses of CanvasItemModelSimple only need to implement the create_item method of the
CanvasItemModel interface, to create the default canvas item to view the item
model.