Improve feedback during message move

[planger]

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.

[rschnekenbu]

Feedback of messages has been greetly improved with PR #449.
However, integrating the figure of the execution in the feedback is an extensive task (See MessageFeedbackHelper class). As time was short, I prefered to avoid regressions on the current implementation.
Moving to Milestone 3.