Fix message flow layout on participant resize

[nikku]

Existing message flows should not break/relayout on participant resize, unless their docking points got resized away.

Cf. forum thread.

Example Interaction

[martinschimak]

Many thx for opening this issue.

1. This is breaking on "move" actions, too.

2. Let me add my personal point of view on the rules for the underlying algorithm. In my mind,

• Message flow's source and target points connected to a flow symbol should always stick at the exact border point of the flow symbol at which they have been connected before. Move and Resize Actions should not change those points by default but prefer a cornered line to connect the points.

  (Reason: Typically I have a very good (readability) reason to connect a flow at a very specific position. When I move some objects, I would get a few cornered message flows, which would not be as harmful as changed connections points. Furthermore I can then decide to move/resize a communicating participant, too, which would typically give me straight message flows again, at least if I find the correct position for the flows.)

• When moving participants, message flow's source and target points connected to this or other's participant's borders should float along the border as long as it is possible to re-connect the messages flow to the participant.

  (Reason: Here it typically does not matter at which point the message flow is connected, and in particular straight lines should remain straight. Therefore the message flows should "float" along the participant's border)

[martinschimak]

I updated my previous comment. Please reread, if you already have read it before. Thx.