sequence diagram usability (#19)

Sequence diagram are now pretty good but there are still some usability issues

- it is not possible to move the label of self messages because you always end up moving the arrow of the message itself.

- more generally it is sometimes hard to select an element, since you often end ud selecting the closest lifeline, of fragment or whatever

- Also it would be nice to be able to add a line break in an arbitrary place in messages label (for example in self messages which do not link two lifeline you don't want their label to be too long)