Reworked edge anchor computation

[spoenemann]

• Moved anchor computation from AnchorableView (that class has been removed) to a new model class SConnectableElement.
• Added SNode subclasses CircularNode, RectangularNode (same for SPort).
• Moved responsibility of route computation from PolylineEdgeView to SEdge.
• Made the properties children of SParentElement and parent of SChildElement readonly.
• Changed signature of views to receive a Readonly<SModelElement>.
• Introduced SModelRegistry, which allows to register model elements via dependency injection.
• Changed most examples to use the new registry, eliminating the need for a customized model factory.
• Fixed configuration for debugging tests in VS Code.