Improve feedback during message move

There is an issue in the visual feed-back during message move remaining stuck at the start or finish of an execution specification. A naïve fix that lets execution specifications have left- or right-side anchors with negative values or values greater than the length of the execution fixes that problem, but doesn't really make sense and breaks a JUnit test for create messages. So, we need to work on letting the `FeedbackHelper` change not just anchor identity but also the connected node figure at each end, which determines the kind of anchor.