Catched up with previous tinker GUI functionalities and new features:
- cannot draw Bang Boxes
- cannot draw Boundaries (not freely)
- cannot draw Goal Nodes (not freely)
+ can draw Edges from/to nowhere (creates boundary as source/target node if needed)
+ when deleting Edge, if source and/or target node is a Boundary without adjacent Edges, it is deleted as well (case where target = source is taken into account)

save, save as, load, new, undo, redo, layout action work as expected