[UI] Introduce base classes for the visuals of standard boxes-and-wires LWFXEF-based editors
Standard boxes-and-wires diagrams using just rectangles and line links
rely on mostly identical visuals. Thus, shared code shall be moved to a
common base class within the
org.fortiss.tooling.base.ui kernel plugin
to avoid code duplication.
(from redmine: issue id 3876, created on 2019-11-28, closed on 2019-12-01)