[UI] Resolve code duplication in the link-method of link visuals
The `link` methods of the `CurvedLinkLayoutedContentAnchorangeController` and `CurvedLinkLayoutedDiagramAnchorangeController` are currently identical. However, as the division in the class hierarchy of anchorage- and diagram controllers comes before the one of lines and curves, the removal of this code duplication by moving it to a common subclass is not trivial. A utility method is not ideal either, as the method contains a `super` call to a protected method.
In the following illustration the code dupliction is between CAC and CDC:
CB / \ AC DC / \ / \ LAC CAC LDG CDC
(from redmine: issue id 3978, created on 2020-03-27)