Convert classes to interfaces

Currently all implementations are classes, but this will be a problem for external implementations of Items and Shapes, so I think they should be converted to Interfaces and actual classes convert (by renaming) them to default implementations.
Example:
public interface GtkCanvas.Item : Object, Actor {
public signal void selected (Clutter.ModifierType modifiers);
(.. convert to abstract or virtual methods)
}
/* Current GtkCanvas.CanvasItem could be renamed to GtkCanvas.GcItem */

Why?

Because I'm implementing GSVGtk with objects to draw SVG images and shapes, so libgtkcanvas can add classes derived from GSVGtk ones and implementing libgtkcanvas interfaces, so it will get SVG shapes for free.