[UI] Introduce base classes for the visuals of boxes-and-wires diagrams

Base classes for standard boxes-and-wires diagram have been introduces
in order to avoid code duplication among similar diagram types.

Issue-Ref: 3874
Issue-Url: https://af3-developer.fortiss.org/issues/3874



Signed-off-by: Tiziano Munaro <munaro@fortiss.org>