DefinitionThe Cgal graphics items hold pointers to Cgal datastructures. When a datastructure changes,
a signal is emitted. An object derived from type Qt::GraphicsItem must provide an implementation of the
virtual slot modelChanged(). This typically triggers redrawing, as well as recomputation
of the bounding box which in turn may trigger redrawing of overlapping graphics items.
This slot must be provided by derived classes.